I need some troubleshooting advice on cabin heating systems.

I am having problems with my 1992 535i. It appears
that the AC runs and will not switch to heat.

I recently converted my AC system to r-134a and the AC
system blows ice cold.

My problems started with what i thought was a bad auxillary fan resistor.
The fuse blew and when I tried to replace the fuse the auxillary fan resistor fried. I replaced the resistor and now the AC runs but the heat
will not come on.

Can someone advise on how to troubleshoot this problem.

I purchased a replacement HVAC control panel on ebay this weekend. Could this unit cause the problem?