The battery in the key died does this mean the car cant be started?

well not crank? or cranks fast but will not start??????
head lamps work, key off and on.?
dead heater blower work on high key on. NO.!
radio dead means battery is totally dead. or a huge fuse is blown. 50 to 100amps , under hood.(rare)
or just loose battery cables. crazy common.
a jumper cable just is next.
battery is not good, none are 17 years old. ever
my life time limit is 10 years on battery life. 1964 to now,
or battery needs a charge
or alternator died and battery died driving.
or car has phantom drain key off and must be found/

ok its a UK car (posted to a clueless usa forum)
with
2.7 diesel Mercedes
this is USA forum
we dont have that engine here, nor books.
is the immo lamp on, cranking
or immo flashing> keyon or cranking?

did you scan the PCM yet,
seems not.

and if not, its a hopeless case.

the glow plugs only glow if the PCM thinks (allen-turing like) it needs to do so based on ambient temperatures or time of last run,etc,

you need to know
you cant fix modern cars lacking this cheap cheap tool.
nor program IMMO parts ever. (dealer only)

i use a $100 INNOVA 3160 on mine,
but not sure if OBD2 works on your EOBD car.
some do... as your country based yours off our
invention of OBD2.
Scantool.net does do EOBD ,(i own one too)

Use the KEY (not the REMOTE) to UNlock the driver's door or the trunk. Then use the key to START the car, then shut it off. Finally, get out and LOCK the car using the mechanical KEY. That should clear the security system codes caused by disconnecting the battery!

When the start for 2-3 seconds and die it's mean just one thing usully , you need to check if the security light blinking and if it doe's you need to call the locksmith to program the transponder key. Locksmith East Aurora Ny 24 Hour Locksmith 716 249 2944 in East Aurora Ny

Try checking the key reader it's usually a plastic ring around the key barrel. If it's dies will not read code in key to enable the car to start. Check your key is sending a code as well.

dirty battery cables or bad battery

If you have a new battery and it keeps going dead, then either you have a bad battery or the alternator is not charging properly. Altenator not charging could be either a bad alternator, loose belt, or bad wiring coming from alternator.

if when you jump it starts its your batt if if keeps dying it your alternator the clicking sound is your statrer which means thats good try the batt when you buy a new one it will have a warranty soo if it dies because the alt you will get a new one batt for free (the alt charges the batt)

try the transmision you may have to replace it

There should be a little plastic cover on the passenger door handle with an emblam of a key on it,underneaththis you should find the keyhole