Don't know how much this is going to help, but might start a ball rolling that ends up with an answer...
Please bear with me
From my rudimentary knowledge of car electrics, the flasher speed is governed in part by the resitance of the indicator circuit.
The lower the resistance in the circuit, the quicker (or maybe slower) the indicators flash.
By changing the bulbs, we
could be changing the resistance by enough to make them flash to fast or too slow (can't recall the exact min / max number of flashes per min that is allowed)
BUT - adding a resistor
anywhere in the circuit
should (I think!) bring the resistance back up to the right value.
So - if the wires at the front are easier to get at, then in theory, it could be added there
Having said that it might be easy to get at the wires in the back anyway