You probably have a leak in the bipressure system. With a leak, the pump
will keep running, trying in vain to pump up pressure (unlock) / suck
vacuum (lock) until it times out (30 seconds on my '87). Likely problem
areas are where the air hoses enter the doors. If you open the doors one
by one and listen carefully as you activate the locks, you might be able
to hear where the leak is.
This is all assuming they still used the bipressure system in '98
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