1987 chevy tbi - wiring switch to dual pumps 
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Welcome to the forum duners.
You have the first year of the TBI's huh?
Which model/eng/trans ?
The 87 GMC Sierra 3500 that I took apart to trace the actual wires
had tan from switch to selector and left pump.
Had gry from switch to selector and right pump.
Red/Wht from left and Red/Blk from right to selector were senders.
Senders from selector to gauge was Red to plug-splice then Pink.
Tanks each had ground for pump and sender of course.
Switch has one of four connections as ground.
So the trick here is that the hot is moved from pump to pump
as the switch is telling selctor which valve to open.
The pump that winds up with two grounds does nothing.
This causes no problems with shorting, just nothing happens.
This leads back to the nature of your question: how do they get power?
The last wire on the switch gets power from first one place, then another.
Follow that wire and after it goes through firewall you will see a plug-splice.
Keep following and you will see it connect at firewall block. (Tan/Wht)
At that point on firewall it splits two ways. Main one is right back at you,
and it jumps into the large loom heading down. Eventually it is the second wire
of the plug-splice that the senders were coming up on.
From that plug it heads back to engine to make 3 important connections.
One is through the fuel pump fuse to fuel pump relay and throught to hot,
as the first way pumps are powered - at the command of ECM(computer).
Second is to the oil pressure switch on engine as the second way pumps
are powered for all but the first 10-20 seconds after starting.
Third connection is a info wire to the ECM, so it knows pumps have hot.
Above it was noted that the wire from switch to power split 2 ways at firewall.
The unmentioned split goes thru firewall main connector there
and on dash side is connected to a module that has a red wire to gauge fuse
and a Blk/Wht ground wire. Has one IC chip, 3 transisters, 4 diodes,
8 resistors, and 3 capacitors. You don't care, long as it works.
There is one more way to get power to pumps, but it is for shop not driving.
When the fuel pump relay is not "turned on", but it is installed,
ALDL terminal socket "G" under the dash can be used to connect
a fused hot wire which will directly power pumps.
Used for trouble shooting certain situations.
Normally when truck is running it's already "hot"
Soo ...........why do you ask ?
And where is your diagram from ?
