The horn and lights flash like an alarm after it was jump started - what could be the issue?

I would check every single fuse in the car. Blown fuses can cause weird issues like this. While the initial issue with the mechanic does sound like an alarm problem....the rest does not unless the shop that installed it really butchered the job.

That's usually what happens when the battery has gone dead and when you are reviving it by hooking up another battery the horn goes off because it is hooked up through the car alarm. Try dissarming the alarm after you hookup the jump leads. That should work.

2 fuses located above relay panel. 1 is for flashing lights, the other is for the alarm horn.

If the alarm light is blinking, then the issue could be with the vehicle immobilizer or key. The key may need rematching to the vehicle's ECU, which will need reprograming by the manufacturer service centre.

The alarm system on your Jeep Grand Cherokee is controlled by a small, approximately 3 inch square box that plugs into wiring under your dash on probably the drives side towards the middle. It is black in color and is printed with the words "alarm module. If you unplug that the alarm system will be permanently disabled and starting issues may be resolved as well.

try proper jump cables and not the cheapo chinese ones ,put battery on charge for about 3 days with a trickle charger to de sulphate the plate and try again

the jetta has a faulty alarm system from the factory i have been working on one in my shop. the alarm moddule is inside the dash behind the light switch and has a 10 pin connector and a 6 pin connector. this can be bypassed if you disconnect both connectors and jump the two large wire on the 6 pin connectors, these wires operate your starter. the car will start and run but your alarm will no longer be active. the door locks and everthing else will still work. the only issue i am having is the factory radio isn't working because it is controlled by the alarm. but worse case scenario i can use an aftermarket radio if need be.

Alarm Module Reset Procedure
NOTE: This information applies to a Vehicle Theft Security System (VTSS) that was armed when the battery was disconnected and reconnected.
When the armed VTSS senses that the battery has been disconnected and reconnected, it enters its power-up mode. In the power-up mode the alarm system remains armed following a battery failure or disconnect. If the VTSS was armed prior to a battery disconnect or failure, the system will have to be actively or passively disarmed following a battery reconnection.
The power-up mode will also apply if the battery goes dead while the system is armed and battery jump-starting is attempted. The engine no-run feature will prevent the engine from starting until the alarm system has been actively or passively disarmed.
Passive disarming of the Vehicle Theft Security System (VTSS) occurs when the vehicle is unlocked using the key to unlock either front door or the lift gate. Active disarming of the VTSS occurs when the vehicle is unlocked by depressing the Unlock button of the Remote Keyless Entry (RKE) transmitter.
Once the alarm has been activated (horn sounding, lights flashing, and the engine no-run feature), either disarming method will deactivate the alarm.
Depressing the Panic button (if equipped) on the RKE transmitter will also disarm the VTSS, but the horn will sound and the lights will flash for about three minutes as part of the Panic feature. The horn and lights can be stopped by pressing any of the RKE transmitter's buttons or driving the vehicle above 15 mph.

need to reset ur car alarm, once battery dies down and u boost her up, the car alarm becomes activated. Just shut off the alarm like u usually do before starting ur car.