If your car is a 2008 VRX so it would be out of the Mitsubishi 5 year standard warranty which makes sense as to why Mitsu are refusing to do it, youd need to claim the problem through whichever company you bought the car from. If they deny it then I say threatening them with the ACCC/Fair Trading w...
Not too sure why you would want to put a piece of black vinyl over your 3rd brake light considering it's illegal? Unless, you're looking to cut out words like 'Lancer' etc. If so, there's heaps on eBay. Anyway, you should be able to source some leftover black vinyl for free from your local sign writ...
As far as I'm aware, if there isn't an 'E-Mark' stamp on the lights somewhere, it is not Australian Design Rules approved. Not too sure if those from Mars Performance are ADR approved though. However, with that being said, there are a fair few members who run aftermarket taillights without any issue...