I've been pondering my options for bringing back keyless entry. Car came with remote keys originally, and the receiver is there.
Option 1:
Original key. The problem is that the system is pretty old, and you can't bind keys, you have to get it with a chip that you install into the receiver. So a key+chip pair from a parted out car, and swap the insides into a key blank. Or a new full key+chip through the dealer (quite expensive). Another drawback is that it's only one button, no possibility to open the trunk. And the killer issue, is that it's a fixed-code system, not unlike the cheapest garage door openers. Trivial to hack with a simple repeater, or even just by
bruteforcing in seconds.
Option 2:
Aftermarket keyless system. Cheapest option, plenty to choose from, rolling codes (so a decent security), will be easy to swap the original receiver box for one of theses. The big drawback for me: can't have the remote in the key, it will hang on a ring, and will bang against the dashboard all the time, ugh.
Option 3:
Lexus key from a later car. I can get transponders with keyblanks from ebay for a reasonable price, and a receiver box from a parted out car. The key is a 3-button, including the trunk opener, which would be great.
This is the kind of remote that was used in later LS400s, but also IS, GS and RX. Quite possibly in many Toyotas too. The issue is that I can't find too much information about how this works. The ads call it "Chip 4D68". I can't even figure out if it's a rolling code system. Also, I'm not sure if the receiver modules offer any analog signal outputs, or if it's just communicating on a bus with a body module, which then controls solenoids. That would be a lot harder to retrofit. And there are countless variations of receivers, different for models, production years, markets...
EDIT: Hm, I guess 4D68 refers to the immobilizer chip, and has nothing to do with the actual remote functionality.