Adam,
On my 2000 Marquis both blower motors were easily accessible under the dash by the driver's feet, and if you have a late 1998 Marquis with dual zone dash A/C, yours should be very similar. However, if you have an early 98 Marquis that is on an Gillig chassis, the fan motor location my be quite different. Also, if you have the standard Marquis dual zone dash A/C system, it has two blower motors, so you need to determine if it is the driver's or passengers motor that needs replacing. Almost any auto parts house will have a motor that will fit.
Gerald