Though not GPS, I remember reading on something called Ravelco. I was going to look more into it at some point, but never got the chance as the vehicle I was going to apply it to needed a lot of wiring work.

^^ I looked into Ravelco a couple of years ago. Interesting and pretty simple but as others pointed out to me it renders you completely SOL if you ever loose any of the specific caps that came with your installation. If I am incorrect about that, someone please say so but that's my understanding of their system. It uses all white colored wires to tap into the ignition and probably other circuits.

It's always sounded like the right system for some people but not everyone.

gerrb, I gave that DSM/VSM-250 GPS kit a look and it's very nice but I have a question: is there any equally good kit that supports the general alarm/immobilizer and GPS tracking functions but which completely avoids the entire remote start and Siri remote start functions?

^^ yes you can just get one with alarm then a separate GPS module . Price won't probably be much different.

^^ I'd be okay with that. I'd prefer it that way. I guess I'd want to stick with the GPS module offered by the alarm manufacturer? I assume they aren't interchangeable or universal.