I think that most of R1a in Germany is Slavic (as well as some Baltic, Old Prussian) simply because of how it is distributed, and because it belongs to the same branches as common Slavic branches. Compare frequencies of R1a among Western European nations and Germans, and how R1a is distributed in Germany:
Western Europeans:
Population (Zeit) ----------- % Haplogruppe R1a (Stichprobenumfang / sample size)
Flandern (heutzutage) ---------------------------- 4,3% (695)
Brabant (heutzutage) ----------------------------- 4,0% (große)
Wallonien (heutzutage) --------------------------- 4,0% (74)
England (vor 1914) ------------------------------- 4,0% (1830)
England (heutzutage) ----------------------------- 4,5% (>5000)
Irland (heutzutage) ------------------------------- 2,5% (>5000)
Wales (heutzutage) ------------------------------- 1,0% (411)
Balts and North Slavs:
Population (Zeit) ----------- % Haplogruppe R1a (Stichprobenumfang)
Kaschuben (einheimische) ------------------------ 63,4% (268)
Kleinpolen (heutzutage) -------------------------- 63,2% (212)
Kurpie (einheimische) ----------------------------- 61,4% (158)
Kociewie (einheimische) -------------------------- 56,3% (158)
Großpolen (heutzutage) -------------------------- 54,8% (343)
Litauer (heutzutage) ------------------------------ 42,2% (301)
Letten (heutzutage) ------------------------------ 40,0% (große)
Germany (regional %):
Population (Zeit) ----------- % Haplogruppe R1a (Stichprobenumfang)
Sorben (einheimische) ---------------------------- 65,0% (123)
Oberschlesien (vor 1914) ------------------------- 63,0% (48)
Brandenburg an der Havel (vor 1914) ---------- 50,0% (14)
Niederschlesien (vor 1914) ----------------------- 48,0% (48)
Ostpreußen (vor 1914) ---------------------------- 45,2% (84)
Graz, Österreich (heutzutage) -------------------- 42,9% (große)
Dessau–Roßlau (vor 1914) ------------------------ 42,9% (7)
Chemnitz (vor 1914) ------------------------------ 40,0% (10)
Dresden (heutzutage) ----------------------------- 32,6% (große)
Rostock (heutzutage) ------------------------------ 32,4% (96)
Halle an der Saale (heutzutage)* --------------- 30,3% (234)
Leipzig (heutzutage) ------------------------------ 27,1% (144)
Mecklenburg-Vorpommern (vor 1914) ---------- 25,8% (31)
Berlin (heutzutage) ------------------------------- 23,7% (232)
Sachsen (vor 1914) ------------------------------- 22,0% (41)
Bern (heutzutage) -------------------------------- 21,4% (???)
Magdeburg (heutzutage) ------------------------- 21,0% (100)
Greifswald (heutzutage) ------------------------- 19,2% (104)
Sachsen-Anhalt (vor 1914) ---------------------- 17,7% (34)
Kassel (vor 1914) --------------------------------- 17,7% (17)
Hamburg (heutzutage) -------------------------- 16,8% (161)
Oberpfalz (vor 1914) ---------------------------- 16,7% (6)
Nord Osttirol -------------------------------------- 16,0% (235)
Köln (heutzutage) -------------------------------- 15,6% (96)
Braunschweig (vor 1914) ----------------------- 14,3% (14)
München (heutzutage) ------------------------- 14,3% (112)
Thüringen (vor 1914) ---------------------------- 13,2% (38)
Unterfranken (vor 1914) ------------------------ 11,5% (26)
Freiburg im Breisgau (vor 1914) ---------------- 10,8% (102)
Lüneburg (vor 1914) -----------------------------10,0% (20)
Schleswig-Holstein (vor 1914) ----------------- 10,0% (20)
Niedersachsen (vor 1914) ----------------------- 9,8% (82)
Lausanne (heutzutage) -------------------------- 9,4% (???)
Bayern (vor 1914) -------------------------------- 8,6% (93)
Hessen (vor 1914) -------------------------------- 8,5% (82)
Mainz (heutzutage) ------------------------------ 8,4% (95)
Weser-Ems (vor 1914) -------------------------- 8,3% (24)
Hannover (vor 1914) ---------------------------- 8,3% (24)
Münster (heutzutage) -------------------------- 7,8% (102)
[Münster (vor 1914) ---------------------------- 9,1% (11)]
Rheinhessen-Pfalz (vor 1914) ------------------ 7,8% (64)
Gießen (vor 1914) ------------------------------- 7,1% (14)
Karlsruhe (vor 1914) ---------------------------- 6,7% (60)
Düsseldorf (vor 1914) -------------------------- 6,7% (15)
Darmstadt (vor 1914) --------------------------- 5,9% (51)
Oberfranken (vor 1914) ------------------------ 5,9% (17)
Baden-Württemberg (vor 1914) -------------- 5,7% (176)
Tübingen (vor 1914) ---------------------------- 5,3% (19)
Rheinland-Pfalz (vor 1914) -------------------- 5,2% (116)
Stuttgart (vor 1914) ---------------------------- 4,4% (68)
Mulheim (heutzutage) -------------------------- 3,4% (59)
Nordrhein-Westfalen (vor 1914) -------------- 2,8% (72)
Koblenz (vor 1914) ------------------------------ 2,7% (37)
Oberbayern (vor 1914) ------------------------- 0,0% (20)
Arnsberg (vor 1914) ----------------------------- 0,0% (17)
Saarland (vor 1914) ----------------------------- 0,0% (16)
Trier (vor 1914) ---------------------------------- 0,0% (15)
Detmold (vor 1914) ----------------------------- 0,0% (14)
Schwaben (vor 1914) --------------------------- 0,0% (11)
Mittelfranken (vor 1914) ----------------------- 0,0% (10)
Bremen (vor 1914) ------------------------------ 0,0% (5)
^^^ The map of R1a in Germany and Austria posted below is based on this data:
http://oi68.tinypic.com/6enpso.jpg
Compare with the map of European languages in 1100 AD (this map is from: Grover S. Krantz, Professor of Anthropology, "Geographical Development Of European Languages", American University Studies, Series XI, Anthropology and Sociology, Vol. 26):
http://s155239215.onlinehome.us/tur...ogrDevelopmOfEurLanguages/KrantzGCh7Fig26.jpg
In 800 AD the extent of Slavic languages was even larger than in 1100 AD, as it also included most of Austria in 800 AD: