1986 gmc sierra classic 305h engine 2wd swb automatic and just noticed that antifreeze is leaking inside the passenger side of truck -is it heater core gone bad?

It's highly likely that your antifreeze leak inside the passenger compartment is due to a failed heater core. The heater core is located behind the dash, and when it leaks, coolant will drip onto the floor, typically on the passenger side.

However, while it's the most probable cause, there's a small chance it could be something else, although less likely given the location of the leak. It's extremely difficult to diagnose with certainty without visual inspection.

To be sure, you should:

* Check under the carpet: Look thoroughly for the source of the leak. You might see staining or even actively dripping coolant.

* Visual inspection (difficult): Accessing the heater core requires removing parts of the dashboard, which is a significant undertaking. Unless you're experienced with automotive repair, this is best left to a mechanic.

* Pressure test the cooling system: A mechanic can pressure test your cooling system to pinpoint leaks more accurately. This is a less invasive method than immediately tearing apart the dashboard.

In short: While it's almost certainly the heater core, get a mechanic to diagnose it properly before you start disassembling anything. Replacing a heater core is a labor-intensive job.