I only know enough to answer part of the question - our 240s are all auto trans.

I'm pretty sure the switch is under the car, and accessible with tranny in place.

I recommend two information sources.

1) brickboard's 700/900 FAQ. Drivetrain is essentially the same, so much of the information there pertains to the 240s. FAQ is available from the dropdown list at top right of these pages. Once you're there, scroll down for the table fo contents. Tranny informtion is in the right column. Or it might be under electrical. Should be there somewhere!

2) amazon.com OR fcpgroton.com
Get a copy of Bentleys manual for 240s. Worth the money. Don't get Chilton's. Haynes is helpful but Bentleys is best. I can't recall which part of fcpgroton's Volvo 240 area has the book, but it's in there.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.