Car Alarms: remote start, horn functions, trunk pop

Question
I have a 2002 Pontiac Grand AM and it has the factory alarm with
keyless entry , dome light, trunk pop headlight and horn functions.

I am interested in adding remote start, and if possible window rollup.
Big thing is that I dont want to lose any factory functions, and dont
want to have to use 2 remotes. any advice on a proper product?  

Answer
Those functions are all achievable with a large number of after market products, but my preference is product made by DEI.  For your particular setup I would recommend a 562T by Valet or the Avital 4200, they are essentially the same system but with different looking remotes.  They both offer dome supervision, an aux output for your trunk release, keyless entry and a horn output with panic option.  You will also need a device to bypass the passlock II system and for that I recommend the GMBP from bypasskit.com, it is marketed under many different brands but this piece allows you to bypass the passlock without the need to cut any factory wiring.