I have a 2009 Mercedes CLK2000 which has not been used for some time. Battery completely dead and cant access the glove compartment to read the car manuals. where is the Battery located I have checked under the hood and in the boot without success

open glove box, read the users guide
if lost it is free from JAG dealer.
it cover all that.
and yes all cars on earth , the battery can be changed
with no damage done. for sure,100%
but you need to read the manual first.
and remove NEG LUG first
first off , last on.
cars with rear batteries have special rules for JUMPS
do not fail to read the manual or damage will happen.
cars with rear battery have a way to open trunk with battery dead, (all do) some are behind rear seat access others a cable latch lever to trunk lock.
SOME HAVE HOOD UP , JUMPER POLES

best is ask here, the manuals are hens teeth now. users. or buy one on line from scalpers.

https://www.jaguarforum.com/search/47893/?q=XJS+BATTERY+DEAD+HELP&o=relevance

Two things to check: Trunk light (may be staying on when the trunk is closed), and the glove compartment light (same thing). Also, disconnect any accessories such as phone chargers, GPS units, and other devices.

After you have driven the car for awhile, let the car sit for two hours, then feel the back of the alternator with your hand. Does it feel hot? If it does, the rectifier diodes have failed. Replace the alternator, and your battery will stop draining.

Fuse box is located in the glove compartment. Empty the glove compartment, then the side will pop out for access to the fuses.

Remote can be matched manually

glove box light,trunk light, it is a current draw overnight
to know if there is a current draw is to connect a ampmeter to battery , ignition switch off, close all doors. if ampmeter read higher than 50mA then remove one fuse at a time when ampmeter change you have the circuit that draw the current and battery dead
hope it helps

Spray the lock with WD40 and work it a few times over you should be able to unlock it.

check to make sure the trunk release on switch in the glove compartment isn't turned off. It's located in the upper left in the glove compartment

perform a battery leak-down test. use a clamp on amp meter and put the clamp around the negative battery cable. the reading should be very low in the milli amp range. if the reading is much higher, there is a circuit draining the battery. with the clamp attached, remove and replace all of the fuses one at a time. when the reading drops you found the faulty circuit. then you have to fix it. OR...there may be a light staying on such as a glove box light or a trunk light or an under the hood light. remove all the bulbs where the light doesn't come on until you open it to see if the battery goes dead.

stick a metter on it to determine the positive .

You can use a slim Jim or get a mechanic to open the door for you. If the battery went dead check to see if your altanator is good and once it's started pull bulb out of glove box compartment and see if that keeps the battery from going dead. Aslo remove the trunk Light to test.