When I was looking to buy or borrow a breakout box a mechanic told me
they aren't necessary and to push a straightened paper clip in along
the wire to measure the voltage on a connector pin.

Sewing "T" pins are perfect for this, sliding right in next to the wire
without hanging on, or risking damage to the seal, can easily be bent as
necessary, and the "T" head works very well with Alligator clips.

A little plastic storage tub of 50 goes for like $3.49.

Erik