I had a problem with the plugs in the trunk on my '99 Grand Marquis. That was not the sockets, but some connectors near the lights. It could be time-consuming to rule out the LCM, but you have to do it considering they cost over $400. if you want connector and wiring diagrams, and connector locations, check over at www.crownvic.net.
The biggest problems with the LCM in these cars is the headlight relays and they can be fixed. Your '98 should be better than some of the mid 00s in that department, but some people “help” them with an external relay anyway. As an added benefit, you get brighter headlights.