Car alarm goes off when I use the key to open doors or start engine on my 2006 GMC Envoy. Problem started when I installed new battery.

SOURCE: 2002 GMC Envoy

If your 2002 gmc envoy or sierra will not even turn over with a kown good battery try starting it in nuetral then go back to park and try starting again. There is a park nuetral safety switch that can malfunction. Turn the key on then shift into nuetral, then turn fully to start.
 

SOURCE: Fuse Box Location 2006 GMC Envoy

Under the rear seat, drivers side.
Black box you will need to remove the cover.
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SOURCE: 2006 GMC Envoy and the A/C fan wont work in number 5 position

replace the resistor 4 the blower motor that should slove that hope this helps

SOURCE: My 2006 GMC Envoy won't start and the battery is good and all the fluids are full, how would I locate the starter to check to see if that was the problem?

ne side starter is mounted against the face of the transmission on oof the engine (usually on the right side) Under the car. There should be 12v at the main lead and 12v at the small (trigger) terminal when the key is turned to the start position. If no power there, check neutral switch and ign switch. You also need to check and verify 12v at the battery and that all connections are clean and secure at both ends of each cable.

SOURCE: rear seat fuse box in 7 seater 2006 GMC envoy denali

Rear fuse box is under the seat bottom, rear seat (middle row) on the driver's side. There is a plastic cover over the top of it that snaps into place.