full of nicotine ...there is no "easy" way to do them in place (and get 'em cleaned "thoroughly")
what Scott said is likely best.
We've never done any, so take this for what it's worth..
if I was doing them, I'd pull 'em , soak 'em , spray rinse 'em
The short ones will fit in a bath tub...not going to happen with the 6 and 8ft's though.
Think I'd consider pressure washing them after a dousing of Krud Kutter .
Obviously you can't use a serious PSI pressure washer...but I'm thinking an electric 1200 PSI might not fold up the fins
Whatever they do to clean them, suggest Krud Kutter for the cleaner...it emulsifies nicotine like NOTHING I've ever tried (that includes many "professional" products specifically targeted for nicotine and smoke cleaning )
BUT..BUT..BUTT...when all is said and done...count the labor hours and PITA factor .
depending on the condition and type of the units, they might be just as far ahead to replace them w/new .
standard "plain Jane" 220vt base board units aren't all that expensive ..like 50-70 bukz for a 6ft unit
..L.T.A.