Flasher modules don't really exist in modern cars. In just about all cars made this century the flashing is done in one the ECUs.
As the OP is in the USA, I assume that this is about the bright red lights that are used for both brake and turn signals.
Almost all incandescent indicators or lights used as indicators are 21 W, so they take about 1.5 - 2A. The OP will need a resistor that takes about 1.5 A, so somewhere around 8.2 Ohms. It will have to be rated at more than 21 W. I would suggest a 50 W resistor. It needs to go in parallel with the lamp.
It's a legal requirement to detect that the turn signals are working. Putting a resistor in parallel with the lamp will stop that detection happening.
Some modern cars will have separate outputs for each lamp, and a higher resistance, lower power resistor may work.
Some combined brake/tail light assemblies will take most of their current from whichever supply is at a higher voltage. If the turn signals work correctly with the position lights turned off, you could try putting a diode in series with the position light wire, which will reduce the voltage to the position lights a bit, and make the assembly take more current from the brake/turn wire when that is on.