I have a to replace an electric wall heater, it's a like for like replacement (except for about 20 years of design!). The problem is that the old heater had it's AC input on the left side and the new one is on the right. The AC line in the wall is not long enough to reach across the back of the wall can through the portal to wire into the new heater. Is it possible to splice this 240V line inside the wall so that it would reach the new power location? If so how?

Use the existing box as a splice box and run cable to the other side, same gauge wire. These will connect to another box, and the ground gets attached.

Use the existing box as a splice box and run cable to the other side, same guage wire. These will connect to another box, and the ground gets attached.

Good, curious if there is a crawl space below and whether the wire comes up from there? Or accessible attic above? Since it's probably dedicated

Is it possible to splice this 240V line inside the wall so that it would reach the new power location? If so how?Absolutely not.
You need to have a box to splice the cable in, and this box must remain accessible, whether it be in the space below, or above, or even right in the wall with a blank finish plate.

Absolutely not.
You need to have a box to splice the cable in, and this box must remain accessible, whether it be in the space below, or above, or even right in the wall with a blank finish plate.

Thanks Stan, that was what I was trying to bring up but also trying not to hijack this thread, if there is a way to pull the line side back into an accessible place and put a junction box there, OP could eliminate having an extra blank covered box in the wall and the circuit could then be extended to exactly the right space