In '99 the 3.8L mustangs had a fuel pump driver module instead of an actual relay, and it communicates with the PCM to determine fuel pump pulse width modulation for the proper fuel pressure (returnless fuel system). Usually the fuel pump driver module can be found underneath the rear (bottom) seat, but i'm not exactly sure about mustangs.
If he's having problems with fuel the first thing i'd look at is fuses, then the Inertia switch in the right side door kick panel, then test the pump before getting into testing the modules.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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