Dont know if its just plug n play with the keypad but
there was someone on here selling a whole unit brand new
for $200 that may help if you get stuck with
parts.

http://www.perth-wrx.com/vb/sale/25689-sale-brant-subarukeypad-imobilser-upgrade.html

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221.7KW-ATW 498.5NM MMS ROLLER DYNO & BUILT BY STi NINJAS, BUILD #318/600

EBISU SUBARU, SHIBUYA-KU TOKYO