Mercedes C180 '96 Heater Fault

https://www.mbusa.com/mercedes/service_and_parts/owners_manuals#!year=2002&class=M-Class
clik on this link for a free download from mercedes for your model series goto pg 250 in the owner's manual for fuse location info there are two locations one is in the engine compartment and one is in the passenger compartment with id info on the hatch of each

we dont have that car in USA, this is a USA forum, so...
plate heaters what on earth are those. Coffee pot heater? Hot plate?

you meant HVAC heater dead. in all modes>?
and is the blower blowing on high? is it?
that is first.
but first first, is engines must be fully hot or there will be no heat.

interior fuse box lower dash drivers side , cover/lid will list the locations

the cab heater fan?, there can be 4 fans on some cars.
front condenser, 2 on rad, and 1 in cab heater box.
the cab fan? does it fail on high too?
it has a switch, (marked fan speed)
the switch is bad ,if low speed are dead. and high are good.
or wiring to it , the motor or the fuse.
even a cracked fuse, change the fan fuse to a new fuse.

Horn and heater may be fuse related. The headrests have nothing to do with fuses. The switch is bad or going that way for the seat or the motors for the headrests are out.

The horn fuse spin it, or replace as corrosion from aluminum fuses cause problems. I suggest replacing them all with new copper barrel fuses. The blower motor fan is a strip fuse on the driver side strut mount in a tootsie roll sized case.

Change the brushes in the heater motor.

Common fault on Merc Vitos and sprinters.

Brushes are on ebay. Search for "heater brushes"

This could be that the actuator that opens the door in the heater is dead.. or the actual heater-core is bad. Which is costly and expensive.

Most likely the switch or the fan motor it self.

these heater control units were in a bad spot..cup holder right below..coffee spills have been known to get in control unit..can be expensive to replace..try a wrecking yard

Heater motor is prone to water damage when the small drain scuttle drain gets blocked (usually by leaves).
If the passenger footwell carpet is damp, that's probably the problem.
Worth checking regularly anyway, and/or remove the small cover over the drain which is part of the problem.
Remove the heater assembly (located over passenger footwell) and replace is the easy answer.
It is possible, depending on ability, to strip and repair.
New from Europarts around £130, used around £60.
Have fun!