Audi put them under the back seat thru the 80's. Not sure if they still do.
Bummer part about putting them there is if you need to do a jump start. Less than convienient.

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)