The ground circuit location is shown in the Wiring Diagrams Manual.
Find the circuit # you are looking for and then search ground locations by circuit #.
Each one of those issues you mentioned above are easy enough to test.
(Other than the radio which sounds like an issue with the radio itself if all speakers work when the radio is on)
1)Turn Signal flashing quickly on one side only is usually caused by a bulb being out, the wrong bulb, a faulty connection between the bulb and socket or socket wiring.
The circuit is designed that way to alert you when a bulb is not lit.
Find the bulb that is not lit properly and determine why.
If you cannot find a problem with any bulbs/sockets (perform a wiggle test to check for loose wires in the socket) then I would have to assume the issue is with the T/S switch/switch connector/headlamp switch.
2)Cooling Fan will not cycle on without the A/C being on.
If the fan cycles on with the A/C then we know the Cooling Fan Relay, Fan Motor and all Wiring are OK.
Sounds like you are not reaching proper operating temp which is probably being caused by the wrong temp thermostat, a stuck open thermostat or a CTS that is not reading temp correctly.
The cooling fan cycles on at 210 degrees F.
Backprobe the CTS connector with a digital voltmeter and start the car.
Watch the voltage, the voltage corresponds to a temperature.
If you are not reaching proper operating temp, replace the thermostat.
When replacing the T-Stat be sure to drain and refill the cooling system properly by removing the bleed plug in the head behind the thermostat (see your FSM for the proper procedure) so you do not trap air in the system.
Also, the thermostat for our cars does not have a "bleed" valve.
You can make one which will help alleviate air and lessen thermal shock on the Head/Headgasket when the thermostat opens.
Drill a 1/8" hole in the T-Stat flange and place the hole at the 12 o'clock position when installing.
Courtesy Of Donavans Dodge Garage
3)The horns ground through the body when they are mounted correctly.
The horn buttons control the ground side of the horn relay through the clockspring.
Do you have continuity from the horns/horn mounting to the body?
Do you have power at the horns when you depress the horn buttons.
If you do not have power at the horns then you need to check the Horn Relay Circuit.
Any problems/questions/concerns Please feel free to ask.