If it's before the air cleaner, use a piece of crazy carpet (or sheet
plastic) and duct tape to prevent picking up a ton of road debris into
your air cleaner.  If it's after, you can do that, but I'd probably make
sure it's airtight or you're going to suck all sorts of crap into the
engine.

Oh, if it's after the MAF (mass airflow sensor) (not sure if a 99 Cav
has one) it needs to be airtight or your car will run lean due to the
extra air - the computer measures the airflow at the MAF.

Ray

That is basically what it does...  Quieten the air noise.  Tape is a
temporary solution and not a good one.  Once the tape comes loose you
are more likely to pick up water from rain splash.  

Steve B.

Even better than duct tape is aluminum foil tape, provided it's the
type with glue having a high temperature rating, around 240 degs. F.
instead of the usual 150F or so.

When I had a duct break last year, I roughened the outside with #60
sandpaper and repaired the hole with fiberglass drywall tape and
sneared silicone rubber sealant (the oxygen-sensor type with no
vinegar smell) into it.

Taking a completely different tack, I decided to try 'welding' the
broken pieces together, using a soldering iron, and scrapings of plastic
(HDPE) as filler.  The joint holds under moderate stress.  Tape will be
plan B.

George

yeah it's just sort of a reverse muffler, to absorb the engine intake
noise before it escapes. like shutting your mouth to muffle a burp vs
just letting it rip.

not a lot of stress there, a good ducttape seal to clean plastic ought
to survive; the el cheapo crap will fall off by itself, of course, not
to mention when wet.