Does anyone know how to replace the rear tail light plastic lens cover without repalcing the whole assembly for a 2003 Highlander?

That is repaired by replacing the lens assembly. There are aftermarket companies that make these for less than the dealer. Check with your local salvage yard, they will likely have new and used options available for you. Used parts in good condition are usually considerable cheaper than new.

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Open the hatch (or trunk if it's a convertible) and peal back the back the trim against the rear most panel. You'll be able to see a lot of large plastic wing nuts. Unscrew them and pull the lense straight out.

try e-bay, I manage to source hard to get items there

Daihatsu Charade EL 2003 rear lens removal. Open the tailgate and remove the two bolts securing the lens assembly. Then get a good hold of the lens and pull out. You need to pull out the whole lens assembly since there are three plastic protruding 'clips' on the side along the lens. Please view attached pictures. Then turn the bayonet bulb holder, replace the slide on type bulb and reassemble everything back in place.

Entire assembly must be changed.

Could be the seal around the taillight assembly is leaking water onto the bulbs. I had a similar problem and resealed the plastic lens cover and that was that.... no more burned out bulbs.

It was very easy. Open the back door to the highlander, remove the plastic compartment cover. You will expose the nuts and bolts that hold the asssembly to the car. Remove the nuts - be careful not to drop any inside the cavity. Once you have all the nuts removed, you can remove the lamps and pull the tail light assembly off. Put the new assembly on, replace nuts, and reinstall the lamps. Easy.

I know the procedure for 2004 highlander. It is as follows...

1. Open the back door to the highlander, remove the plastic compartment cover.
2. You will expose the nuts and bolts that hold the asssembly to the car.
3. Remove the nuts - be careful not to drop any inside the cavity.
4. Once you have all the nuts removed, you can remove the lamps and pull the tail light assembly off.
5. Put the new assembly on, replace nuts, and reinstall the lamps.

usually one removes the inner carpeting behind the taillight revealing the fasteners

thode screws hold the lens or the light assembly? Allow for the whole tail light to come out and the bulb plug will be accessible. I never had the privilage. But how the lens fits will tell you. If the lens goes overthe edges, the whole unit comes out, lens is sealed. Good luck