You could use a $10 relay
SOURCE: Need a wiring diagram for mastercool evap 2 stage unit
I have the wiring diagram in the cooler. I can photograph and send it to you. Do you still need it?
SOURCE: I have a DuoTherm BriskAIr 13.5K BTU. The Dometic
If you look at control box at roof unit, they usually use a separate set of wires for furnace. See what colors they are attached to at harness to thermostat. Some just follow other harness, and come out at thermostat. (bypass control box) Once you've determined which 2 they are, join them together & see if furnace starts, as it should. If it does, thermostat is faulty. If not, pull outside cover off & check voltages at furnace. I'm guessing by furnace model that your unit is approx 30 ft , but if it's a park model, you may have 24 volt system on furnace or 12 volt. Another way of confirming is to go to back of furnace where air intake grille is inside your unit, and locate 2 blue wires going to furnace & remove whatever wires are attached to them & connect them together. Furnace should start up.
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OK if anybody cares.. the answer is yes. You can convert a two wire system to a 4 wire system if you have both a 4 wire control board (the 624625A in this case) and a 4 wire t-stat. Of course you have to have 4 conductors in your cable going from the t-stat to the control board. I wired it up today and it works fine.
And one more comment, the reason I did this was that the 624567 2 wire control board is becoming scarce and the price has jumped from $112 in early 2011 to $268 in mid 2012. The 4 wire control board I used for this conversion can be had for about $65. That leaves plenty of cash to go buy a 4 wire thermostat and still be ahead about $140
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