Re: "Failed" lights warning symbol on dash.
I'm not sure I follow you.
Initially the warning came on only when you braked. Did you change the brake bulb?
Now you say that the warning light is on all the time and goes off when you put the side lights on. If the warning lamp only is on when the lights are off then it's not the bulbs......normally people forget to check both rear number plate ones or the two parking bulbs in the middle of the headlamps.
Check the drivers side rear light - is it glowing dim all the time when the lights are off?
It sounds like you need to firstly check the connections on the back of the rev counter (remove the screw and it should come out - there are some clips holding it though). It's quite possible that you need a new rev counter as that's where the detection circuitry is for the lights.