Intermittent vehicle speed sensor malfunction

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I've had my Octavia for about 18 months now. The speed sensor failed so I purchased another. While replacing the sensor I noticed that the guy that had the car before me had run a supply from the junction box on top of the battery to one wire of the vss harness. Upon removing this wire and testing the supply to the harness it was only getting 3.2v and with his wire it was getting 14.5v. I put this wire back on and put in the new sensor. Everything worked fine for around one week and the sensor went again. I seen online that someone else supply to the sensor was 14.21v so I removed his supply and took a supply from cigar lighter which was 14.23v and bought a new sensor. Tried the car and everything works fine for a few hours and then the speed sensor stops working again. The next day I'm driving and the speed sensor starts working again so I wonder is there a bad ground? I checked/cleaned the ground cable from battery to the body under the battery cover and it goes from there to the engine but i can't find a ground strap going from the engine to the chassis. Is there one I'm not seeing? Any help is much appreciated.

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I'd just like to add that now my diagnostics is reporting multiple sensor faults.

Heater control circuit sensor 1 and 2.

Evap purge control valve circuit sorted.

02 sensor heater circuit.

Vehicle speed sensor range/performance.