i have a 1991 mercury grandmarquis what kind of mods i should put on my car?

Tint the windows.

Update the stereo.

License plate frame change.

Rims if you have the money, otherwise new caps.

Get it detailed.

Maybe change the headlights to a different "temp".

It depends on how heavily you want to modify it, and your budget. Are you looking for bolt ons, or to supercharge it.

1991 was the first year of the 4.6 in the panther platform, so you have similar options to the mustang crowd in terms of bolt ons. If you're mechanically inclined, you can look into a PI (Perfomance improved) head/intake/cam swap. The PI engine is found on 99-04 mustangs and later model explorers/mountaineers and crown vics, etc.

Sounds to me like you're just looking for bang for your buck bolt ons. Not much you can do with the marquis, but you might want to look into aftermarket gears. You have a stock rear gear ratio of 2.73. An upgrade to 3.73 gears will really wake up the car, but will hurt your highway gas mileage a little. This doesn't add any power, but makes it feel much faster. Think of a bike, it's like shifting the back gears to the bigger ones.

Also look into a K&N filter. That'll help a little

a decent stereo, keep it clean, and enjoy the cruise. the girls always prefered my cruisers in school compared to the little impossible to make out in models. besides you have the bass compartment there is.

I have a 99'. I'm thinking fuzzy dice =D