in beiden Fällen 1.)
2.) ist falsch.

But the contracted form ("Where's ..." and, similarly, "There's ...") may be heard followed by a plural subject in the casual speech of even some well-educated speakers.

a. particularly if the plural subject refers to a single object or a single set
"Where's my glasses?" (single object)
"Where's my gloves?" (pair)
"Where's my keys?" (set, or "ring of")
--and perhaps less frequently if patently plural
"Where's my books?"

b. also where there is a series of subjects, and more particularly if the first of the series is in the singular:
"There's cake and cookies in the conference room."
"There's two more people still waiting outside."

"Where are" and "There are" contract somehow awkwardly and when they are contracted tend to sound indistinct.

What gmp says is true -- it's a very common mistake -- but it's still considered a mistake, a sloppy usage, and it's certainly one that's more common among less educated speakers. There's really nothing all that hard about 'Where're' and 'There're,' and those are definitely the correct forms, even casually.

As for the original question, I don't think there's normally any ambiguity about 'Where're my glasses?', because in real life, no one is likely to be searching for more than one pair at a time. The same goes for scissors, pants, gloves, and so on.

In some other context -- say, you had brought six pairs of plastic Groucho glasses to a party, complete with nose and mustache, and couldn't find any of them when you were ready to go home -- you would probably add the word 'all': 'Where're all my glasses?'

One more thought: There might indeed be some ambiguity; around here, when (as frequently happens) an older person asks 'Where're my glasses?', the usual answer is 'Which ones? Your reading glasses, your computer glasses, your driving glasses, your sunglasses?'

Not that I should talk, I'll probably be there soon myself. (-;