OK Guys... THIS NEEDS A RELAY! For one it trips a fault in the GM5 that the siren wire is short circuited. These piezos may not be enough current draw to do permanent damage but connecting a real siren actually makes the windshield wipers go off when the alarm is triggered! The siren wire is a logic wire designed to run a relay in the factory siren, not to sink the current required to drive the alarm. Thats why that brown ground wire is in the harness. I just picked up a $5 SPDT "PC relay" from radio shack to fix this install on my brother's car.
Photo in Cleveland, Ohio
1999 323i - Totaled at 265,000 miles.
Now an e30 & e90 owner.