WHERE IS THERMOSTATON 03 DIAMANTE - 2003 Mitsubishi Diamante

Follow the radiator hose to the engine block. Remove the bolts replace the thermostat replace the bolts.
https://www.youtube.com/watch?v=Ue6R4SiztWk

Battery problem or main fuse blown

The reverse lights are operated/controlled by the neutral safety switch located on the transmission close to the tranny dipstick. This switch is adjustable and may correct your problem with a slight adjustment. Here is a picture that may help.

check for faulty reverse light switch

1995 Mitsubishi Diamante - Kelley Blue Book

www.kbb.com/mitsubishi/diamante/1995-mitsubishi-di...
Kelley Blue Book Review by Tommy - ‎Jul 29, 2015
1995 Mitsubishi Diamante overview with photos and videos. Learn more about the1995 Mitsubishi Diamante with Kelley Blue Book expert ... See all specs.

1997 Mitsubishi Diamante - Kelley Blue Book

www.kbb.com/mitsubishi/diamante/1997-mitsubishi-di...
Kelley Blue Book Review by Mohsen - ‎Mar 23, 2015
1997 Mitsubishi Diamante overview with photos and videos. Learn more about the1997 Mitsubishi Diamante with Kelley Blue Book expert ... See all specs.

Hard to find parts for a Damante, I'd call around junk yards for the control arms. You can probably find an aftermarket replacement for the struts at a parts store. Take them with you so you can find a reasonable facsimile.

Worst senerio head gasket..But make sure the caps good and no resrictions in any of the hoses

Assuming your Diamante is constructed similarly to my 03 GTZ you will need to remove your tail light assemblies in order to replace the bulb. To do this you will need to remove the carpeting on the rear wall of the trunk to access the nuts holding the assemblies on. Remove the nuts on the inside of the trunk and the assemblies will just pull off from the outside giving you access to the bulbs.

Only Recharge the Battery with both the Cables: Positive and Negative removed from the Battery.
With the Charger OFF the Battery - Reset your Alarm turning it OFF.
Then Disconnect the Battery completely.
Never Recharge a Battery while it is still connected.