IIRC when you blow that tunnel up, that should be where the ending credits appear.
Are you running any mods that might affect HH? Also, a couple of times near the start of HH, when scripted parts of the story didn't trigger with the Happy Trails expedition, I found that simply waiting 1 hour put the game back on track. That might work. Or you could try going back to an earlier save (hopefully not too far back) and replaying that end battle/sequence. svcks when junk like that happens.