The Volvo Service Manual Wiring Diagrams has the schematic diagram that is copied by Haynes and Bentley, but in 3-color printing and thus easier to read.

That WDM also contains a lot of other things, like a pictorial of every system, and a chart on cable harness connectors, relay locations, and ground connections.

Warren is correct about the leads from that well-known gray connector. The pictures shows #1 in the lower right position.

#1 SB Oil pressure sender. Easy to re-route
#2 Y Temperature gauge sender. Also easy
#3 R Alternator D+/61. Easy and critical.
#4 open
#5 BL-Y Starter motor terminal #50. Easy (#5 is the lower left in picture)
#6 open
#7 open or BL in some models
#8 BN Starter motor terminal #16.

Caveats: (1)This is assuming you have the B23F engine, not a D24 or B21FT. (2) The removable side of that connector can be opened up. There are little doors on each side. The connectors can be attached onto the new wiring.
(3) Do not cross the two connections on the starter, or it'll never even crank. Check before removing the old wires which one goes where. You may have one wire there, or two. Depends on whether your model has a ballast resistor. I had to remove the heavy cable in order to get at the small wires. (4) I have bought this manual for all of my 240's, about $40 a pop, and it's absolutely worth it.

Good Luck,

Bob

:>)