So the EGT and trans temp both quit working at the same time? Do they do anything, or do the pointers just stay down at zero? Are they the EV series, with the amplifier box under the dash?
Definitely check for voltage right at the amplifier box.
Please also do this quick test: Pop the gauge out of the pod, and check voltages across the opposite terminals (there are 4 in an x-pattern). Let me know what you find, and that will lead us to the next step.