I have decided to purchase a Robertshaw thermostat model 9701i2 that requires a common wire for continuous backlighting and my Trane Air Handler model 2tee3f37b1000a does not have a "C" common on the terminal wiring block. My existing thermostat is a straight forward RC, W, Y, and G 4-wire setup. I know I need to add another wire but I am unsure of where to connect it on the Air Handler.

The existing wiring to the thermostat is pretty straight forward except for the RC (24V Hot).

Here are the connections as they stand now. Thermostat is on the left:

W-W1
Y-Y
G-G
RH (red wire) goes to a safety condensate pan switch. Out of the switch is a white wire that connects to a condensate pump. Out of the condensate pump the red wire goes to the R terminal on the Air Handler. ( I am thinking that the Condensate pump has a transformer in it?)

There is also another 2 wire cable (not sure where it is going) that has a white wire connected to the B terminal of the AH and the red wire connects to the Y terminal of the AH.

Any help would be much appreciated.

This should give you an idea:

Thermostat wiring code

It's the other side of the control transformer that doesn't go to the thermostat.

Thanks! Great Explanation.
Graphics were just what I needed.
I'll give it a try this weekend.