I have a canned post on bypassing the relay with a short jumper at the relay plug.
You could just use a longer "jumper" wired to a dash switch. You'd still be using the normal Fuse 11 voltage and the original under-car wiring, if that's a concern.

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To bypass the OD relay, replace it with a jumper wire at the relay harness connector plug by connecting these two relay harness plug terminals together:
• Input terminal #15 (2 wires, a Blue and a Gray)
• Output terminal #87 (White wire)

A good-fitting jumper can be made with flat male crimp-on terminals that are the same size and shape as the relay terminals. This jumper wire will simulate a relay that is always energized — which is the normal condition, until the shifter button is pressed.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.