Heating of the compressor looks like an add-on to me.
http://i46.photobucket.com/albums/f1...Fall2010-1.jpg
The two white connectors at the bottom hook up to a heat-wire that wraps around the compressor's base.
There is a short coil spring mechanically connected to heat-wire end loops, to keep it tight.
The 18k BTUh version of this unit, (AC/HP) does not even use this heater.
My point is, you might be able to add a crankcase heater for cheap money.
That is, if you even needed a heater.
If you find a pdf manual online, please post the URL for us..
And of course you could post some pics with the top cover off..
If you could be so kind..
Thanks,