Alternatively, if you’ve shielded the cavities, you may have a signal wire grounding on the shielding.

A reading of 0 ohms indicates a short between signal and ground. It may be a wiring connection error or it may be a loose strand of wire bridging contacts or it could be wiring touching shielding. Faulty pickups would normally be open circuit, so give an overload or overrange reading on your multimeter.

As Wierdy says, check the output jack connections. Being a TRS ‘stereo’ type, a standard TS instrument jack will short the R(ring) and S(sleeve) connections on the output jack, so if you’ve wired to those instead of the T (tip) and S, you’ll get a signal short.

I always double check barrel jack connections using an inserted TRS jack cable and a multimeter as on some jacks, the difference in length between the two short tabs is minimal. The physical arrangement of the pins on a Switchcraft jack tends to have the central connection as T, the outer one as S and the one in the middle as R, but other makes can vary. So I find it always best to check myself.

You can link the R and S connection together to provide a bit of extra contact security for your ground connection.