this was an enjoyable undertaking for me. i had electronics shop in high school (loooong time ago) so i am dangerous with a soldering gun. i actually used a $29 jcwhitney keyless entry unit with built in dual relays. i figured out how the relays were wired, took them off, modified the PCB and put them back on. with that approach i didn't need additional external relays. i hooked up in the trunk following that second link. kept me engrossed for a couple of days! it's a great DIY project but i might upgrade the unit to one that as a chirp or beep confirmation at some point....
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"