Windshield will fog like hell in a gen 1-2 due to shitty design. Gen two has a really nice carpet pad over plastic on some trim levels that hides leaks. 
Harder to see than you think. Same firewall 83-92, so hides like mine did. Took me a week to figure out and I didn't say anything on here because I was worried about a head. 
I'm with bobbyD, I don't think it's heater core. Why's it pissing occasionally? 
Heater cores on first gen was OK ish design. Not grounded, so that's a win. 
Yes, with electrolytes in coolant, it will oxidize your heater core if it's grounded. Sort of. Electrolysis does spooky stuff. My electro tank is evidence. But it mauls your shit. Replacing hers could be preventative maintenance only. 
Before spending money, have advance or Napazone or Irish mcpartstore pressure test your cooling system. Get the results for us. We will decipher the runes... 
Temperature affects results. Block may leak only when cold or only when hot. 
Better to save the cash if it's gonna be something big.