Horn and brake lights

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.

All except the brakes sound like fuses failing, or wires breaking. A quick check of the fuse box should remedy those.

What do you mean by "the brakes"?

does the pedal go to the floor without stopping the truck?
do the brake lights not come on?
is the brake indicator on the dash on?

need a little more to go on

I'm sorry to say you need trained help or a good DIY neighbour to get to the bottom of this.; The ABS sensors on the rear brakes will not lead to air in the brakes or grinding of the brakes. It sounds like you have an ABS pump fault in the main unit. This can cause the brakes to hang up after releasing the pedal. The rear wheel ABS sensors may only need their mounting positions cleaned up and their location to the brake reset, this can be as little as 1-2mm. I'm sorry for your financial situation but you are going to need help and my advice is don't drive the car until you have it fixed. Your life or someone else's is in your hands.

The problem is with the brake lights, there is a solenoid which prevents you taking the vehicle out of park unless you have your foot on the brake, the solenoid is wired into the brake light circuit. Bridge the two wires to the switch and see if the brake lights come on, that eliminates the switch, as the horn has stopped working as well the chances are it is a fuse. To test a fuse, remove it from the box and use a multi meter set on resistance and check for continuity, if they are all ok, you are looking for a wiring fault! You could try using a live fused fly lead from the battery direct to the brake light switch to put the brake lights on, and see if the vehicle will come out of park. Then it's just a matter of finding the wiring break? Try all connectors. Good Luck

all I can say is that real cold temps are cars.Minor problems can turn out to be large ones.I\'d put car in a garage and let it warm up,and test it.

I've seen it happen many times on the older Caravans, if you hit a really deep puddle, the splash can knock the accessory drive belt off of its pulleys. You can just gently pry the belt back on, consult a service manual for detailed installation directions.

Could be either one or both get the battery charged if you have another vehicle hook up cables and see if it takes a charge for about 30 min DO NOT CRANK THE ENGINE FOR ANY REASON JUST PRESS YOUR REMOTE TO SHUT OFF THE ALARM SO IT DOESN'T GO INTO THEFT MODE AND PREVENT STARTING WHEN I WANT YOU TO START IT OK THIS IS VERY IMPORTANT THAT YOU FOLLOW WHAT I'M TELLING YOU OTHERWISE YOU WILL DRAIN THE BATTERY FROM CRANKING IN THEFT MODE ///////NEXT AFTER THE 30- 45 MINUTES OF CHARGING THERE SHOULD BE SUFFICIENT POWER TO START AND RUN THE ENGINE MAKE SURE THAT YOUR BATTERY HAS DISTILLED WATER IN IT IF YOU HAVE CAPS? ON IT BEFORE YOU START THE CHARGING PROCESS NOW FOR A SECOND OR 2 HONK THE HORN AND SEE IF YOU HAVE HEADLIGHTS A LOUD HORN AND STRONG LIGHTS WILL SHOW A CHARGE IF YOU HAVE A VOLT METER CHECK FOR VOLTAGE BETWEEN 12.5 TO 13.5 VOLTS IF YOU HAVE THE VOLTS THATS GOOD NOW START THE ENGINE IF THE BATTERY LIGHT IS ON IN THE DASH THE ALTERNATOR MAYBE BAD CHECK FOR VOLTAGE AGAIN WITH THE METER IT SHOULD BE 14.4 IF YOU DON'T HAVE A VOLT METER QUICKLY DISCONNECT THE BATTERY CABLES WHILE THE ENGINE IS RUNNING IF IT DIES THE ALTERNATOR IS BAD THE MOTOR SHOULD KEEP RUNNING OFF THE ALTERNATOR POWER THAT IT IS GENERATING SO GOOD LUCK I'M SORRY IF I LISTED THINGS OUT OF ORDER I'M A TAXI MECHANIC IN NYC WITH 50 YEARS OF EXPERIENCE BUT I'VE BEEN WORKING FOR 16 HOURS TRYING TO GET CARS RUNNING FROM VERY HEAVY RAIN HERE TODAY THE DRIVERS AREN'T SMART ENOUGH TO SLOW DOWN WHEN GOING THROUGH FLOODED ROADS AND HUGE PUDDLES AND I'M BY MYSELF THE OTHER 2 GUYS ARE OUT WITH THE SUPER BOWL FLU SO I'M TRYING TO HELP YOU ON MY LUNCH BREAK AND IN BETWEEN VEHICLES SO PLEASE READ EVERYTHING FIRST PUT DISTILLED WATER IN THE BATTERY FIRST IF YOU CAN OTHERWISE CHARGE IT AND DO THE REST DESCRIBED OK GOOD LUCK STEPHEN

No there not connected, replace the brake light
1999 Chevrolet Blazer Auto Light Bulb Size Guide
Low Beam Headlight Bulb Size: 9006
High Beam Headlamp Light Bulb Size: 9005
Parking Light Bulb Size: 3157
Front Turn Signal Light Bulb Size: 3157
Rear Turn Signal Light Bulb Size: 3057
Tail Light Bulb Size: 3057
Stop Light Bulb Size: 3057
High Mount Stop Light Bulb Size: Assembly
Fog/Driving Light Bulb Size: 880 or 881
License Plate Light Bulb Size: 194
Back Up Light Bulb Size: 3156
Front Sidemarker Light Bulb Size: 194
Ash Tray Light Bulb Size: 161
Glove Box Light Bulb Size: 194
Map Light Bulb Size: 168 or 211-2
Step/Courtesy Light Bulb Size: 168 or 211-2
Trunk/Cargo Area Light Bulb Size: 211-2
Under Hood Light Bulb Size: 93
Instrument-General Light Bulb Size: 161 or 37 or PC194 or PC74
Check Engine Indicator Light Bulb Size: PC74
Hi-Beam Indicator Light Bulb Size: PC74
Seat Belt Indicator Light Bulb Size: PC194
Brake Warning Light Bulb Size: PC74
Automatic Transmission Indicator Light Bulb Size: 194
Directional Signal Indicator Light Bulb Size: PC74
As for the horn, inspect the fuse and relay and most common it would be a bad ground wire or worst a bad horn due to corrosion. Horn will be located in the drivers side fender.
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check your battery cable isnt loose

the bulbs are a dual philament eventhough the tail lights work the brake light part of the bulb is burnt out