The BMW heater is made by Webasto. If BMW doesn't sell a kit for post-sale installation, then go to Webasto.

Here is the UK link, complete with a listing of local dealers.

http://www.webasto.co.uk/am/en/am_automobile_3177.html

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue