I presume (probably shouldn't do that) but... I presume you have a 302 (5.0 litre) in the car???? The easiest thing to do is go to www.autozone.com, enter "repair info" area and put in the required info for your vehicle. I have found this to be a good resource for a number of questions.
NO belt in the 302cid engine of a 1988 Mercury Grand Marquis. it is a chain with gears
troubleshoot head lamps on 1988 grand marquis? Not working...fuses OK
where is the fuse for tail lights in 1988 mercury grand marquis
( 20 U.S. gallons ) according to my Chilton's book ( for a 1988 Mercury Grand Marquis )
According to my Chilton's Auto Repair manual : A 1988 Mercury Grand Marquis has a ( 20 U.S. gallon ) gas tank
The 1988 Mercury Grand Marquis circuit breaker for the power windows can be found in the fuse box. The circuit breaker will be in the first column, third from the bottom.
The stereo wiring diagram for a 1988 Mercury Grand Marquis is available from a factory service repair manual. These manuals can be found online and PDF. format.
have someone sit in car and spray it down with a garden hose to find leak
No. Same displacement but different compression, horsepower, etc. The Mustang engine had 220hp; the '88 Grand Marquis had 140hp (or 150hp w/ facotry dual exhaust).
I believe that is the 302 cubic inch ( 5 liter )
fuse box under steering wheel
1988 Grand Marquis hubcaps have a lock on them. You'll need to buy it. The center panel (with the mercury logo) comes out and you'll see a little opening that is flower shaped. That needs to turn to release the hubcap from the lugnuts.