I have succesfully retrofitted supplemental heater (ptc) to 2.0 tdci mk2 focus. I tried searching older posts but didn't find anything about retrofiting ptc, so here is some basic info about this mod.
What is ptc-heater?
-About 1kW electrical heater element
-Heats only interior, NOT engine
-Fitted on some diesels, mk2 onwards
-Located on heater assy, possible to install without removing dashboard
-Temperature controller (manual or automatic) sends signal to GEM when max heat is requested
-If ambient temp is low and engine is cold, GEM commands PTC with PWM signal to operate. PTC has 3 resistor elements which are used all together or one at a time, depending on required heating power.
You can test GEM before even buying ptc element:
-Use Elmconfig to configure HEC and GEM to support PTC heater.
-Wait for a cold morning.
-Start engine and set heat to MAX. If ambient temperature is below 10 (at least in my case) and engine is cold, instrument panel should give you a message: Aux heater 100%. If you get that message, GEM supports PTC and all you need to do is install it.
-Remove trim panel from footwell side.
-Remove bolts 6 and 7.
-If footwell light socket is installed, it has to be popped out or reinforcement element 8 won't have enough room to come out
-Carefully take reinforcement element out of the way
-Remove ptc heater cover retaining screws 10
-Dismantle ptc heater. Remove screws from the plastic cover, open cover and remove all the wires. Put cover back with few screws to cover electronics. This has to be done because element won't fit unless wires are removed or dashboard is removed.
-With a litlle force, bend the plastic panel in front of ptc installation hole and slide the element in place. Element has rubber nipples at the other end and heater assy has holes for them. Needs a little patience to get them right.
-When element is on its place, unscrew the cover and refit all wires.
-Reinstall reinforcement element and attach all wires.
-Reinstall trim panel
It's actually quite efficient, warm air comes after about one minute after starting engine. Difference is huge, tdci is quite slow to warm up.
I don't see any reason why it could not be installed in petrol cars, all the important parts should be the same between diesel and petrol.
EDIT: More specific installation instructions and few pictures