Viper and Bulldog make very good products. I also found this site a few weeks ago while looking for alarm parts and acc.... http://www.vehicle-security.com/keyless.htm

No prices, but the DYNATECH Co. (as I have been told by my friends who are into electronics) is responsible for manufacturing most factory-installed vehicle security systems..... worth a look.

Good Luck,

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William
76 240D - 550K miles
78 300D - 200K+ miles