Did they bring back Salma for it?
No, but they do get a lot of cameos and share a lot of in jokes from the whole Quentin Tarantino/Robert Rodriguez film universe. And there are a lot of good looking women in it, along with lots of campy violence. Here is the episode that started this season -
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It's a bit hard to describe last season, but at the start of this one, the Gecko Brothers have escaped, and are forced to separate, with Ritchie, now a vampire, wanting to marry Satanico(Selma Hayek's character from the film). Seth goes with the preacher's daughter from the movie, who wants to make it back to the Titty Twister to save her brother from the whole vampire cult that is based there.
Selma and Richie are being chased by the rest of the Titty Twister vampires due to a bunch of infighting between rival clans, and they all are also being chased by a young lawman, who's partner was killed at the start of the series.
It's a bit complicated, but it meshes together decently if you can watch the whole series.