[BrendaAnnD]

Measure the current draw between the positive terminal on the bridge and the place it connects into the radio. Multiply the voltage (25) by the current you get in amps (ex. 50mA = 0.05A), this would give you a result indicating a 500 ohm resistor rated at at least 3 watts (actually 1.25W, but always go up a bit for safety and longevity.)

Your calculations will vary of course, since 50mA was just an example in this case.