GM-GMC: Rear heater blows cool air in 1999 GMC Diesel Suburban 1/2 Ton, gmc diesel, lack of circulation

Question
Can't find my original question listed on the site.

Most of the time recently my rear heater controls blow only cool air.
Wondering where to look for possible solutions. Truck was in body shop
during April 2008 and many things have not worked properly since. Thank
you.

Following is the reason:

What was done to your truck at the body shop?  Do you feel they are
related?  If so, why.

Expert: goodwrencher

The left fender was replaced due to minor damage; bumper was replaced.
Left battery cover and other parts were left off when the truck was returned
to me. The block heater cord was left hanging down to the ground and
ultimately a mechanic yanked on it and caused it to malfunction. Has taken
me three dealers and three left mirror replacements to finally get the
electrochromic mirrors working again. I no longer take my truck to the
original body shop; they returned it to me without making all the repairs
and left multiple parts missing. I don't feel I can trust them and would not
be surprised at anything they might have done in order to get a customer
back--thinking I'd be foolish enough to not realize they had created new
problems. None of these problems existed prior to the body shop work. The
front blower and heater works without any issues; the rear heater blows air,
but it is cold. I wonder if they might have disconnected something.

Thank you.  

Answer
It is possible that something was disconnected or it could be coincidental.  I am not sure where to even tell you to start on this problem.  Wiith your truck at full operating temperature, reach underneith and check to see if the heater hoses running to the rear are hot.  If they are not hot, follow them back up front and see if you can locate a blockage or any reason for the lack of circulation.  If the hoses are hot, move to the inside of the truck and check to see if the rear blower motor, mode door etc are hooked up and function as intended.  They are located in the rear quater panel, you will need to pull a panel of two to access them.  That should get you started.
Hope this helps