oil filter and pressure lights on dash?

[TomLS]

How do they complete a circuit then? Mine only have 1 wire going into the back.

[bmcecosse]

If you look - they both have two wires. The Red ignition light operates between battery +ve (via Ign switch) and the alternator - and so glows whenever either is at a higher voltage than the other. So - no charge = light glowing, charging = no glow. The Orange oil pressure warning light earths out through the switch on the engine block which is normally closed - so the lamp glows when there is no (or low) oil pressure, but extinguishes when the switch opens as the pressure rises. Thus neither of these warning lamps relies in any way on an 'earth' connection on the speedo. The headlight main beam, and the 'illumination' bulbs do of course have only one supply wire - and earth through the speedo.