you can do this on your envoy VERY easy... if you get a CAN network Bypass module... one wire from your OBDII port will give the module access to talk to the BCM and control the locks lights arm disarm factory alarm system...
Flashlogic - FLCAN
http://cdncontent2.idatalink.com/corporate/Content/Manuals/DL-GM1/FLC-AL(DL)-GM1_20111004.pdf
If your a DEI person they have something like this as well. not sure if theirs does the seats tho but i know this one does. I just picked up my 06 Trail Blazer LT 5.3 and i will be using an Audiovox APS997C and a FLCAN bypass module with this combo i will only have about 7 wires or so to tap into on the truck and it will do it all... lock , unlock, start, panic button, 2-way radio key fob, use factory key fob for remote start ,heated seats and rear defrost. (will have to run other wires for the shock senor programing switch and so on)
For under $200 you can have it all.
(by the way i made an account just to tell you this... i have been reading up a lot on adding in a remote start to my truck using the CAN system Google search landed me to this forum post )