I have a 1999 Wellcraft Excakibur w/ a 502 Mag.
I have an alarm that is going off about 10 seconds after a start it up. It doesn't go off, just rings solid and not stop. I just replaced the thermostat, but plugged the two sesonsors back in.

Any ideas?

The alarm will only sound for low drive lube, Low oil psi, or high water temp.
You probably have a bad sensor for one of them.
Start unplugging the sensors one at a time. When it quits beeping, you found the one causing it. I have also had a spice in alarm system wire come apart and when the wire would hit the intake manifold, it would ground and set off the alarm.

Thanks. Where is the low oil psi sensor? I know where the others are

Either on the back side, on top of the engine next to the distributer using a tee, or most cases it's down low on the right side facing the front of the engine down on the bottom where the oil filter would be (remote oil filter connetion).