battery is gone or needs to be FULLY charged.

also have the alternator tested. taking a look at the infamous trunk hinge wiring harness wouldn't hurt either...
http://www.lexls.com/tutorials/lighting/bulbout.html

 

It sounds like the alternator how big is your battery charger you need a charger that can charge 35 amps per hour and starter amps at about 200amps used only to start the car. I bought one at Kragen for about $150.00 on sale. I also noticed that my 91 lexus will go thru batterys about every 2-3 years. I am a industrial power supply engineer and we make power supplies that will charge batterys IT TAKE ABOUT 3 HOURS OR MORE TO GET A GOOD CHARGE ON A BATTERY AT 35 AMP HOURS. IF YOU USE A 10-15 AMP CHARGER LEAVE IT ON FOR ABOUT 6 HOURS TO CHARGE YOUR BATTERY. IF YOUR START YOUR CAR AND EXPECT THE ALTERNATOR TO CHARGE THE BATTERY IT SHORTENS THE LIFE OF THE ALTERNATOR BIG TIME ACCORDING TO THE REBUILDERS OF ALTERNATORS. THE ALTERNATOR WILL CHARGE THE BATTERY IN ABOUT 3 -4 HOURS OF DRIVING AND IF YOU NEED TO DO THIS. THE LEXUS ALTERNATOR IS A 100 AMP UNIT

If you need to charge your dead or near battery using the car alternator it will take 2-3 hours of driving to get it charged up well. This is very hard on the alternator due to the load heat and running all the other electrical parts on the lexus. computers fuel pump etc. if you have a small charger 10-15 amps leave it on over night it will charge the battery just fine saving the alternator. HOPE THIS HELPS

Have your battery checked for a dead cell.