02 minivan ?

The RKE module is under the left side of the dash top cover. It is a two wire module, with a B+ connection, and a single wire to the BCM.

I don't know if you can hack an aftermarket unit into that system, the data the RKE sends to the BCM is the issue. Your local car alarm / stereo shop may know that answer.

You will have the dash open to get at it if you replace it or hack it. Probably just easier to put OEM parts back in

Do you have the Sentry Key as well?

B.