I've bought a couple of Zerostarts on Ebay for about $10-15 and they continue to work well. I think it was from a seller who must have bought boxes of old stock. They had obviously been sitting on a shelf for years.

http://cgi.ebay.com/ebaymotors/Zero-Start-model-150-600W-1-1-2-engine-heater_W0QQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247QQcategoryZ6763QQihZ025QQitemZ380087172642QQrdZ 1QQsspagenameZWD1V

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13