I install a aftermarket radio and it work fine until I connect a ground located under the steering wheel. When I disconnect it radio works, what is it,please help.

Speculation without further details but 3 usual causes:
1. Clock spring in steering wheel for airbag open circuit.
To Test - disconnect battery-disconnect airbag ribbon from steering wheel -insert 3 -8ohm resistance bypass in airbag plug below steering column feeding clockspring. reconnect battery and see if light goes out after 5 seconds
2. Check passenger seat sensor operational.
To test - with ignition off - disconnect plug under passenger seat for sensor pad and insert test ohm resistance in feed plug. switch ignition on as test 1.
3. Check steering wheel button operation. if any radio actions do not work problem may be infrared link between steering wheel and radio (especially aftermarket modules)

You have all the informations to find your problem. jump a wire on connector X4 G1 tan wire to ground and check X1 E5 with a test light or multimeter to see if there is a output, you should have 12 volts. If you dont have any voltage the horn relay is defective. I check in the shop manual and the relay come with the underhood fuse block.

OK, that radio use a straight or constant 12 volts a ignition 12 volts and grounds. now your radio ground might be connected to the park light wires of the car. that wire will should grounds ( - ) but when you turn on the park light it turns to 12 volts to illuminate the dashboard gear shift etc. if that is the case you're wondering where you will get grounds to collect the radio, well the antenna wire has ground, so plug and play, and the frame where the radio goes has metal, which is also grounds. disconnect the black wire from the radio and that wire you can test by turning the park lights on and you will get 12 volts

Almost sounds as if the steering wheel wire harness is disconnected!
All those things are bundled into a harness that is connected at the factory when the steering wheel is installed.
The connection is probably inside the car under the dashboard.
Be blessed.

LOOK UNDER THE LOWER AIR DEFLECTOR FROM BELOW THE RADIATOR. IF YOU HOT WIRE CAR HORN AND IT WORKS.YOU HAVE TO REPLACE THE WHOLE AIRBAG IN STEERING WITH HORN CONTROL BUTTONS.YOU NEED TO DISABLE AIRBAG IN STEERING WHEEL WHEN REPLACING AIRBAG AND HORN CONTROL BUTTON PAD.JUST TURN OFF RADIO FIRST, THEN DISCONNECT REMOVE THE NEGATIVE BATTERY CABLE.WAIT 20 MINUTES BEFORE WORKING ON AIRBAG AND HORN PAD IN THE STEERING WHEEL.

If you take the steering wheel off you'll see the ground wire that grounds the constantly connected horn. Note the color of the insulation.

You could also reroute the wire from under the hood starting at the horns and simply connecting a switch that grounds the horn out to operate it with an outside switch, that is outside of the steering wheel.

Replace the clock spring under steering wheel, if applicable to your vehicle

when I had that problem it was the contacts under the steering wheel pull the steering wheel and you will see a copper ring the horn is connected to ground out that copper ring against the steering column if the horn sounds you have a worn contact I fixed mine by putting a spacer under that copper ring good luck with it

If you disconnect the stereo, does the temp. display go back to working properly? Did you connect the stereo Illumination wire correctly? Are all the other stereo wires connected correctly? Double check what the wires in the aftermarket harness are supposed to be and don't just go by color.

probably the clock spring uner the steering wheel. It transfers voltage from the steering column to the steering wheel.

Section 11-04: Steering Column 1997 Villager Workshop Manual
REMOVAL AND INSTALLATION Sliding Contact, Air Bag Removal

1. WARNING: THE BACKUP POWER SUPPLY ENERGY MUST BE DEPLETED BEFORE ANY AIR BAG COMPONENT SERVICE IS PERFORMED. TO DEPLETE THE BACKUP POWER SUPPLY ENERGY, DISCONNECT THE BATTERY GROUND CABLE (14301) AND WAIT TEN MINUTES.
   Remove the steering wheel (3600). Refer to the procedure in this section.
   

1. Match mark and tape in place the inner and outer elements of the air bag sliding contact.
   

1. Remove the two steering column shroud screws and the lower steering column shroud (3530).
   

1. Loosen the shifter boot and position the upper steering column shroud out of the way.
   
   
   
   
   

1. Remove the three air bag sliding contact screws.
   
   
   
   
   

1. Disconnect the two lower air bag sliding contact electrical connectors.
   

1. Remove the air bag sliding contact from the vehicle.