Have a 3 switch to heater light and fan. when i turn on heater with light on dim, flash/spark behind heater switch. is this normal

Wiring is incorrect.
Sounds like the wiring is in series instead of parallel.
http://waterheatertimer.org/How-to-wire-switches.html#triple

Gene
Water Heater Timers Save Money

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Could be your alternator is getting too weak to keep up with demand of the car's electricals. Have your alternator tested, or have a shop check your charging system. Either one will test the alternator. Know anyone with a voltmeter and how to use it?

I'm not sure what is going on without full and complete description of wiring and switches:
Add a comment with exact information for possible help.
Specs say 20 amp circuit is needed: so 20 amp circuit breaker + 12 gauge wire are required.
Heater uses 1500 watts, so it pulls a lot of power if same circuit breaker is used for several other rooms and a TV etc. Turn off everything else and see if problem persists.
Sell sheet for product says: Energy efficient lighting - uses one 27-watt (max.) high efficiency GU24 based fluorescent lamp.
Turn off power, and disconnect ground wire. Turn power back on and switch on the heater-lite and see if there is a fault to ground.
Use a different switch.

It may just need to reset itself. Turn the Master On/Off switch to OFF and unplug the unit. Wait 12 - 15 minutes before plugging it back in. Then turn the Master ON/OFF switch to ON and set the heater as you normally would. That time out, should give the controls time to reset themselves.

Hope this helps you solve the problem.

check your wiring it sounds like it is shorting out.
If it is new you should have the wiring diagram.

The wiring harnesses in the plugs behind the bulbs are burned up seperate rear of headlight cover and inspect parts available at your ford dealer

Replace the fan motor AND blade . ( evaporator fan motor ) . This motor is located inside the freezer , behind the rear panel . The switch , is a fan/light switch , when closed , turns on fan , and light off . When open , light on , and fan off . normal operation .

Here is the ignition sequence for a spark ignited furnace. This will help you understand what is happening/not happening.

Thermostat calls for heat.

draft inducer motor (DIM) starts drawing draft through heat exchanger.

Pressure switch detects proper operation of DIM, and sends signal to board to begin ignition.
(it detects negative pressure)

Ignitor starts ignition attempt and gas valve sends gas to burners.

Upon ignition, flame sensor detects flame and sends signal to board stopping ignition sequence and continuing gas flow for heat mode operation until thermostat is satisfied.

During this sequence the blower motor is brought on, and by using a series of temperature limit switches, the system assures that the unit is operating properly.
(If the blower is not operating, or the airflow is inadequate, the heat exchanger will overheat and trip one or more temp limit switches.)

There is also a "rollout" switch that detects improper flame pattern "rolling out" into the controls compartment.

Now, to apply that to your situation.

If you have no spark, than the failure is happening before the gas is supposed to be flowing.

That usually means a failure at the D.I.M. Typical causes for that are a clogged flue pipe/vent, damaged/deteriorated fan blades, bad fan, or bad pressure switch.

If one of the temp limit switches or rollout switches have tripped, it will simply shut off the system.
(Some units will turn on the blower motor and shut down everything else.)

You can try resetting the unit by disconnecting power for 1 minute, and attempting a restart.

First look for any flashing LED lights. If you see any locate the wiring schematic (usually on the inside of the front panel) and look for the flash codes.

If the reset does not solve the problem, let me know what flash code you are getting, and/or let me know exactly what you see the furnace doing, in detail, after the restart.

Did all these problems happen all at one time?? If so you may have a bad body ground. Try running a electrical wire from the (-) negative side of the battery to the body and see if this works. Be sure this wire is to a place with no paint as low voltage electricity will not flow through paint.
A). If not try replacing the turn signal flasher for the turn signal problem. This is usually the culprit and it's easiest and cheapest to replace. The turn signal flasher will be found under the driver side dash next to the hazard flasher. Turn the hazard flasher one to locate it. If you want try swapping the turn signal flasher with the hazard flasher to see if this fixes it. They are the same style flasher.
If this doesn't work you may a faulty turn signal switch or some wiring issues.
B). It could as simple as the head lights bulbs need to be replaced, it's rare but I have seen when both bulbs blew at or near the same time. If your able check the voltage at the head sockets.
C). Could be a bad blower motor or switch. I'm not sure if these models have a resistor block or not. The resister block controls the electrical current going to blower motor which controls the speed of the blower fan.