Its standard to use a heater from a mini in a sumo build. This has the advantage that it's a simple, small unit and fits in nicely, but another option is to reuse the heater from your ford donor. This has the advantages of being free (assuming you have access to the donor) and is a far better heater than a weeny little mini heater that, won't warm your cockles on a chilly day when your doing 70mph with no roof on.
The problem with the Sierra heater is that its way too big to fit in the space behind the dash. Ideally the heater will be fully fitted behind the dash (Its an IVA requirement not to draw air into blowers from the engine bay). To fit it all in the heater needs a bit of work to get it to form a single unit (its split in the donor) and get it to fit in the place for the mini heater, but nothing you can't achieve in a couple of hours and the end product is well worth it.
Once fitted behind the dash, you can easily control the fans with simple switches. There is really no need to have a cold air blower in a cobra, so you won't need any valves on the water to turn the hot water flow off, you either have the heater on, or you don't. (trust me, I fitted water valves to allow cold air and it was a waste of time and space)
Starting with the complete unit, discard the ducting and all the controls. LEAVE THE WATER INLET/OUTLET PIPES ALONE THOUGH!
The blower is a nice compact unit and remains unaltered, but we do however need to fix it directly onto the matrix unit so it can go under the dash.
Start by cutting the matrix unit so that you lose all the shaped ducting parts at the front and some of the housing, it should end up looking something like this:
Next, we need to fix the blower unit to the front of it. To do this, make a simple wooden template by drawing round the blower unit back fixing face.
Next, with a generous amount of sealant and some bolts, you can fix the 2 together. Remember, it doesn't have to be perfectly sealed as you want the warm air out anyway and there will be no flow over it when the blowers are off, so you won't get unwanted warm air on a hot day.
Now the 2 parts can be fitted and bolted together.
In order for most of the warm air to either go into the footwells or into the demister vents, we need to seal the matrix in and only let the air out where we want it. This is done with some simple bits of hardboard, glued, bolted and cable-tied over the openings you don't want.
Once you've added 2 bits of wood and sealed it up, a lick of black paint and it looks rather nice……just like a bought one.
Now its just a case of fitting it. The 2 pipes we left on the back can go through the bulk head ready for a couple of water hoses, you will probably need to relocate the 2 holes in the bulkhead for these.
You will also need a couple of very simple brackets at the front the fix it down (you can see these in the photos at the front bottom of the heater unit) and also connect the ducting up to the screen vents. As you can see in the picture, I left another opening next to these outlets to let a nice bit of warm air into the foot wells. Tip for the ducting - garden centres that sell pond pumps and fish will stock plastic pipe that is exactly the right size for the ducting and its very cheap indeed. :D
The wiring is very simple, there are 3 speeds and you can use as many of these as you like, I choose to have speeds 2 and 3 only available via a couple of nice old style lucas rocker switches on the dash.