When I created the remote doorbell computer starter, I did not think of using it with a car... there might be a way getting the button in a keychain size.
anyway, for your remote needs, lets look at this guide:
http://www.modasylum.com/guides.php?guide=16
its way more complete then the latter one specified above.
I could suggest you the UIRT/2 for remote AND poweron. Its all indicated in the guide, with tons of URL.
hope that helps.