My Mitsubishi Magna, has just had the battery replaced and the car won't start. On instaling the old battery, the battery light shows while the engine is running but the heater, stereo and rear stop lights won't work.

I think you should buy repair manual first for your vehicle after that it will be easy for you to solve the problem. Check the link may be you will find something useful here
Complete Mitsubishi Magna 1998 2001 TH Series 1998 2002 TJ Series Verada...

There are Magna workshop manuals readily available to show you how to do this work.
Buy one of them.

Hi good people, My TE Magna has been suffering the immobiliser no ... NO you can not bypass the immob, a gound or power will NOT give the ECU the correct code from the immob that it requires ... 1992 Mitsubishi magna.
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dont know what the heck the other answers were trying to do fix
T he question was how to remove the starter motor on a 4 cyl magna 1988 mitsubishi .. that makes it a TN or first gen magna

ok TN can be either carby or an EFI setup
disconnect battery
14mm bolts two hold the starter on
undo the battery positive cable on the starter

and the ignition wire on the starter

On an EFI you have to go under the engine and undo the support bracket of the efi inlet manifold to get the starter out
underneath the car
the interim shaft is one of the bolts holding the bracket to the block
on a carby you can do it from the top
from the drivers side

You need to have the Alternator tested, the battery symbol indicates the system voltage is low. The other problems are due to the low voltage.

Malfunction in the charging system. Have your alternator tested, it's probably bad. Do it soon or your battery won't have enough charge to start the car.

When you changed the battery the computer must re-learn the setting again. If it continues have a OBD II reader plugged in and check for codes.

code search at auto zone and they can sell you the parts to fix it

if both headlights are out, probably blown relay. user manual should show where it is located.

first of all you should always use a wire harness when installing a unit into a vehicle you should never cut the factory harness but guess you already did so here is the wire diagram for your car hope this helps http://www.installdr.com/Harnesses/Mitsubishi-Wiring.pdf