re: trade in car or sink money into it...

This is not the same thing but I paid to have the transmission rebuilt in my 2005 Altima earlier this year. I paid about $4000.00 but the car gave me virtually no problems before that so I sunk the money into it even though it was only worth $7000.

It would have probably worked out for me had I not totaled it in an accident a few months later. It was very minor damage but the value of the car is what totaled the vehicle.

My only point is planning on the unexpected should factor in as well. Will you waste your money if something else happens to your vehicle - a major breakdown, accident, etc.

Since your car is worth $12k on a trade in you are probably fine fixing it.