Judging by the look of the unit and the "D3L" markings I
think this may be an older Eberspacher Diesel Heater:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?
ViewItem&item=2400322756&sspagename=ADME:B:SS:US:1

See what you think. The heater is currently sold today by
Espar:

http://www.espar.com/htm/Specs/D3LCspec.htm

Thanks,
Chris