300TD Fast Blinker, Where to start?

[veggie190]

My guess is a fast turn signal means a bulb burned out. Thing is every light looks fine when testing. One of the rear license plate bulbs was out, I replaced it but the fast blinker remained.

Any advice on where to start? Is this most likely a burned out bulb? Is there something else to check? Any tips on checking the lights, in the rear I don't want to ruin any trim.