2003 elantra new battery, won't start, no lights

Here is the fuse box diagram, but the Elantra appears to have a rather funky battery fuse! Most vehicles have a main battery fuse but it is on the battery connector wire. If you put in a new ECM, was it programmed to the vehicle?

https://www.autozone.com/batteries-starting-and-charging/battery-fuse/hyundai/elantra-gt

Hyundai Elantra (XD; 2000-2006) fuses and relays

Fuse Box Diagrams https://fuse-box.info > hyundai > hyundai-elantra-xd-...

The fuse box is located on the driver's side of the instrument panel behind the cover. The location of the fuses in the passenger compartment: Hyundai Elantra ( ...

Have u checked the main fuse in the car? Should b like an 80amper, good luck!

look for main fuse panel under hood, there should be a charging circuit fuse check it

could b the altenator/regulator,they check em @ advanced for free

After connecting battery, try using key in drivers door to lock, and /or unlock. Try passenger if that doesn't work, as well as valet key if it came with one.

Look and see if it has some kind of an alarm

Pl. make the battery terminals firmly tight.Hope this might help.

clean the throttle body

As you can see in this wiring diagram, power supplied is series/parallel

Sounds like you have a bad or burned contact in the ignition switch.