hi,
It can be done, BUT its not very elegant.
A 15W lamp at 12V passes a current of 15W/12V =1.25Amp.
To drop from 12V to 6V [ie; 6Volts] at say 1.25A = 4.8R
The power dissipated by the resistor would be 7.5Watts,, it would be HOT.
Between the 12V car battery and the lamp you need a power transistor, mounted on a heatsink. The base voltage of the power transistor would be controlled by a standard variable resistor.
A transistor is DEFINATELY the way to go. so simple to integrate. I would suggest a NPN Power transistor. If you want to continue, lemme know and I will draw you a schemmy of it to help you.
aside from building simple subwoofer crossovers and i only have basic knowlage of transistors etc. any schematic is welcome. all i want as aformentioned is just to have and inline variable manual control of the bulb (varying the value between 6v-12v)
Well a 2N3055 power transistor would work fine and is an easy cheap transistor to find. However like any power transistor, you will have to mount it to a heat sink of sufficient size.
As far as the rheostat I would prefer to see a fixed resistor, to limit the maximum current draw, wired to the pot for decreasing brightness from that set maximum. I'm sure someone here can 'spice' simulate the resistance values for you.