2004 Buick rendezvous

no crank no start.  fuel gauge on dash goes up to a
qtr tank when ignition is in on position, but after it
sets for maybe 20 or 30 secs all of a sudden the
gauge drops to empty.  Now a lil history " customer
brought the vehicle to us says all of a sudden he
got in it and tried to crank it and nothing happened
but day before was driving fine.  now I've checked
the battery it was good can crank it with shorting
out the starter but dies right after it starts up. have
checked all the fuses and relays all are good. Can
hear the fuel pump kick on when I turn key to the
on position. now there is no security light on but try
to reset Security just in case and still nothing. Got
power to everything on the inside and outside
except for starter engaging. Lights on dash go off
when you turn to start position just like they should
and come back on when ignition goes back to run
position. Now there was a aftermarket radio in it
but it was running before when aftermarket radio
was installed and ran afterwards for months. I'm at
a loss pls help we have had these car for two
weeks checking things that we think it might be
when we go on break in between fixing other
customers vehicles. pls help