COMPRESSOR ISSUE or 2ND TANK

I have a 1998 Ford Expedition XLT 4.6 V8

I have tried charging the AC and it only blows cold in the back and blows heat in the
front of the truck at the same time. Every so often, the front will blow cold for 5
minutes and then back to heat.

Heard that it was a compressor issue but compressor works/runs. Heard I had a 2nd
air tank for the front of the truck but don't know if its true or where it to find it. So, can
someone give me a hint as to where this second tank would be located or if the air
compressor is just in need of replacing. 2nd tank would be so much cheaper but
whether it is realistic or not in an expedition is another story.

Thanks!