So, do I need any adapter kits to install the wireing harrness? Also do I have to install a computer module since I am replacing the head unit?

 

I think you might want to get it installed by professional when it comes to video/nav units just my

 

i put one in my neon srt 4, wasnt hard at all, there is alot of stuff you have to fit in the dash, there wasnt that much room. but i bought a harness and it plugged right in. but not sure about how the radio is integrated in the hhr. i put tvs in the seats of my neon too, was all easy, just takes time.

 

I put that unit in my first HHR..
At the time the Lan harness wasn't available..

Use the Harness and should be fine..