Jeff,
The part numbers that you listed for my 2000 Marquis are also applicable to your 99 Marquis. The brake pads and rotors are the same on all 4 wheels, so wheel location does not matter. The brake pad number that you have is for one wheel, but Meritor now sells a set for both wheels (4 pads) on an axle also, but any good truck parts house will be able to cross-reference the numbers. These brakes have been out of use on new applications for years, so you may need to wait a few days for the parts, but there are a lot of tour buses, fire trucks, and RVs that used them, so parts are readily available.
Gerald