Volts, amps and watts are all in relation to each other. Higher wattage at same voltage equals higher amperage.
Not saying that's your problem..... Just saying
If voltage is a constant and you put in a brighter bulb ( higher wattage ) you will increase the amp load of the circuit
When the higher wattage bulb blows put in an even higher wattage bulb. Then when the fuse blows put in a higher amp fuse. Eventually the smoke will come out somewhere else and you will find the root of your problem.
Check the license plate lamp wiring harness. You'll have to completely dis-assemble rear door panel to access and remove. As well as rear power lock assembly to gain access to housing where license plate lamp housings reside. Start there...I'd make a very confident bet that is where the short is.
just a thought, behind left side tail is also the birthplace of trailer wiring harness. I have encountered a pretty nasty PO ghetto connection, that caused me some similar problems.
if all else fails, you might have a look there.