Some very good points have been raised in this thread. I'd like to add my inflated two cents.
B&M tests every siren before it leaves the shop for function and current draw. Our test bench has a very accurate ammeter. Your siren is guaranteed to draw less than 70 amps. Actually, it draws 60 amps at full wail with an inrush current (or startup current) of 150 to 170 amps. 6 gauge cable is recommended for runs under 15 feet, 4 gauge for 15 to 25 feet. Trust me on this, I know the guy who owns the company. Never use a cable smaller than 6 gauge as undervoltage burns on the commutator will eventually develop, severely shortening the life of your armature.
Compare these figures to a Q, which draws 120-125 amps maintenance current, 350-375 inrush current. Federal absurdly recommends 4 gauge cable which is only rated at 95 amps continuous use. Fortunately, ambulance and fire apparatus builders know better and universally use 2 gauge, rated at 120 amps.
If your lights dim severely it's a safe bet that your body feed is deficient. Start at the source: Batteries. Not battery, batteries. When you add a real siren, you join the Varsity and you'd better have the juice to run with the big boys. Your batteries should be matched and replaced as a set. Dissimilar batteries will charge at different rates. Battery terminations should be cleaned and inspected regularly. Remember, battery posts are not the only battery terminations- there's another end on each cable which also needs to be cleaned and inspected. Cables themselves should be adequately sized and in good condition. Cables with clamp-type replacement ends are not in good condition. Also, remember that corrosion can form on the cable inside the lug end as well as the battery end. For this reason I make all of my own battery cables using very high quality components. I have to disagree somewhat with Todd as I never use cranking circuit cables under 2/0. In fact, the usual cranking circuit cables I make are 4/0. Vehicles with 300 amp alternators get 350MCM cable. You don't find that at Auto Zone.
Feeds for any non-Pontiac accessories should come directly from the batteries, not from the Pontiac body feed on the starter solenoid. That body feed terminal should also be inspected , as it is often the source of troubling voltage drop.
In order to sustain the load of warning lights and siren a serious alternator would really help. The battery lead from the alternator should be sized to match or exceed the output of the alternator. If your car has the stock Pontiac alternator and wiring harness (judging by your description of your amp gauge, it sounds like it does) your batteries will discharge markedly in the shortest of parades. A high quality volt gauge is a better indicator of electrical system condition than an ammeter, by the way.