Here is an approach. You have two signals, Turn (red trace, which is at +12V when the turn signal is on), and Side (green trace, which is at +12V when the side marker lights are on).
If you connected an XOR gate to those two signals as shown in the "conceptual model", you get the output V(Lamp), blue trace, which mostly does what you asked for. When the side lights are off, the wheel-well lamp follows the turn signal. When the side lights are on, the wheel-well lamp is on during the time when the turn signal is off. Note that leaves the wheel-well lights on after the flashers are cancelled, as shown after 13.5sec in the simulation.
The circuit shown is a "power XOR gate" which works directly with no logic gates. R1 are your wheel-well lamps wired in parallel. The PFETs M1 and M2 have to pass the wheel-well lamp currents, and so do the Schottky diodes, D1 and D2. The parts are not critical, but should have voltage ratings of >30V. The diodes will dissipate some heat, so should be mounted on a suitable heatsink. The PFETs should have an on-resistance of <50mΩ with a gate drive of -10V