I think most heater matrixes with out the casing are small enough to hide some where in the chassis, you only need to have a small enclosure to heat passing air. I thought about putting one on the back of the passenger foot well/bulkhead, that way if you have any problems it's only bonnet off not dash and scuttle off. You could always make your own from copper 8-10mm fuel line if you have a radious bender and a few hours to kill. Having said that I've just had a look on fleebay and found this for not alot of money.
http://www.ebay.co.uk/itm/Micro-Heat...item2eb29cb45d
http://www.ebay.co.uk/itm/PC-Liquid-...em20bc3f dcfb
