Did a quick hook up with and without resistor, noticeably brighter without, more than photo shows. Not nearly as bright as brake/tail (no photo)
The issue is the switch. They would have to have designed and built a higher current switch to handle the current required minus the resistor. Also the internal circuit breaker would have to have been rated for more current. My recommendation is to leave this the way the engineers designed it.
Yes, that makes sense especially if they used a mustang h/light switch (not sure) for 6 bulbs. I was thinking of a resistor grounded at one end causing more load (like another bulb) but inline would reduce current. There is also the 6 amp circuit breaker back there to prevent the resistor wire from burning (fire) in case of a short in the bulb area because the one in the switch is rated higher I’m sure.
Thanks to everybody, it’s nice to learn new things about a car I’ve had for 45 years. I was 5 when I bought it… 