I know this is an old post ... but ...
But, anyone ever find out what happened to this guy? Is he chained up in the basement?
After quickly reviewing all the advice you folks gave him, I think I have a different slant that could have helped him. It's an angle no one else seemed to have thought of.
Okay, first of all, he's already taken holy vows to this woman, in front of God and everyone. So, I don't think kicking her to the curb is an option. And, clearly, by his own admission, she is in charge. So, the key is to make her want the Marauder.
Pitch it as the ultimate dominatrix-mobile. A Marauder (well, at least a black one) exudes power and sexiness. Make her think how powerful she'll feel driving it. Plant subtle seeds in her mind that will eventuate in her "commanding" you to pay for mods to the car.
Of course, he'll be left driving the 94 Grand Marquis. I didn't say my plan was perfect.