I like powershop's suggestion. If you want electric heat, there are a few things to consider. First of all, it would take a whole bunch of amps at 12V. To give you an idea, there are hair dryers that give off heat in excess of 1200W. Don't know if Wifey's hair dryer would keep your cab warm, but that's 100A if you could power it at 12V. Next thing is HP to run the oversize generator / alternator. It would take more than 2HP of engine to produce the amps.
Mebbe you could have a "Summer/Winter" hookup for the hydraulic oil cooler. A three-way valve setup to circulate through the existing fintube out front in the warm weather, and another fintube with a small fan blowing heat in the cab in Winter? I'd bet my B7500HSD produces enough hot hydraulic fluid to warm a cab...............chim