I've only been out of bed for about (looks...) 15-20 minutes, and haven't had any coffee, yet, so maybe I'm missing something Really Obvious: But please explain why positive grounding vs. negative grounding (or "negative switched" vs. "positive switched," as you'd have it) is any easier to troubleshoot with a simple incandescent test lamp
Ok. Assume you are living in an age when multimeters aren't for your average layman. All you have in your test kit is:
(a) a piece of wire with bare ends.
(b) a 12VDC light bulb.
Let us take a mast light for example: It isn't working. Okay. If the switch is in the negative lead, with the switch 'off' you should have +12VDC all the way from the battery, through the light and back to the switch. Test procedure is then:
1. Connect test bulb between +ve & -ve at panel: If it lights up, we have volts.
2. Connect test bulb across switch contacts: If it lights up, the mast bulb and all wiring is good.
3. Use piece of wire across switch contacts: If mast light lights up, switch is faulty.
4. At the top of the mast, connecting your test bulb from either side of the mast light to a known ground (eg. the stays), completing the circuit the test bulb should light. If it only lights one side, the mast bulb is blown; if neither side, the feed wire is open circuit.
With the "positive switched" system we have now, there is no +12VDC available at any point in the system downstream of the switch, so the only way to test the circuit for continuity is to use a multimeter set to "ohms" (effectively a battery and an ampmeter in a box) - or hook a test bulb up to +12VDC and have it light up on everything it touches, working or not... but there is no doubt that this system is safer.
Does that make sense now??
EDIT: The above assumes the system is still negative grounded
. Perhaps the confusion (I'm confusing myself here!!) is coming from the idea of changing from negative to positive grounding. IIRC, positive grounding was used to minimise arcing - not to assist with fault-finding.