Hello i have a seirra/c1500 2wd ext cab. Anyways I had an after market radio installed a while back and ever since then i do not have defrost. I do get heat but it doesnt switch over. Anyways we are checking it out and i was googling for a heater diagram but was unable to find one. Does somebody have a blowup of the heater control unit that shows the vaccum lines and what not? I almost went into a snowbank because of the defrost not working. Please and thank you.