I installed a keyless entry / alarm on our 190E, and the complete unit with starter disable relay and two three-button remotes was $49. Additional parts required were a pair of Bosch style relays for triggering the pump, diodes for the parking lights, wire, tape, heat-shrink tubing, solder, etc. A cheap DIY compared to pro installation, but that is partly because it is a lot of work. The other advantage to DIY though, is that YOU get to secretly choose where stuff is located, and a thief is less likely to be able to guess how to disable it.

I also installed the same system on our Accord DX (and THAT also needed 4 door lock actuators installed), and the receivers from both cars can be programmed to the same remotes. Didn't do that, since the cars are parked next to each other at home, but could be a useful point for some.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7