simple alarm system, relay timer?

I would like to make a crude and simple alarm system for my vehicle (98 Chev Blazer).
Here is how it works:
There is a toggle switch located on the outside of the vehicle (hidden somewhere). This is what activates the system. If the switch is closed (on), and the dome light comes on, the horn will blow. If the switch is open, the system is off and nothing happens.
The reason i want to use the dome light circuit is because it comes on whenever the door is opened of even if the door handle is just pulled up.
There is also a "test switch" located on the outside of the vehicle, which, when pressed, illuminates a small light bulb to say if the system is activated or not.
I know this is a pretty crude system, but I already have most of the parts required left over from other projects. And I probably wouldn't use this much, only when I park it in certain places.
Please see my diagram below.
I was wondering if someone makes a timer that I could put on this circuit, so that the horn would only stay on for 5 or perhaps 10 seconds?

Also, my vehicle has an aftermarket remote entry/starter system that was already on it when i bought the vehicle. I don't know what brand it is and all the writing has worn off the remote. I was thinking about using the rear hatch release button to activate/deactive the system. I think I would just cut the wire going to the lock solenoid on the back door, then re-wire the solenoid to the switch that is on the dashboard. I would use the wires that used to go to the lock solenoid to activate/deactive the alarm system. But I don't know if this type of switch exists. I need something that, when given a voltage, opens a switch. Then when given the same voltage again, closes the switch. Or is there a better way to do this?

But most importantly, I need to know if there is a sort of timer that I can put so the horn doesn't stay on for more than 10 seonds (or so), even if the dome light is still on.

THANKS

Here is the diagram:

just to clarify, when i said "activate the system" i should have said "arm the system." The toggle switch controls whether or not the system is armed. If the toggle switch is closed and the dome light comes on, the horn will blow.
The test light is just an extra feature.

Hmm, a few thoughts here and there:

--You probably know that a basic alarm system is fairly cheap, right? From a financial point of view, this project is hardly worth the time.........but if you're doing the project just for your own enjoyment, that's cool. I like projects too.

--The horn wire, inside the car, is a low-current ground trigger---it'll be one of the black wires at the steering column on a GM car. So, you shouldn't need that second relay at all.........just have that first relay, so that the ground from the toggle switch passes through 87 30 ONLY when the door is open..... that'll activate the horn.

--Two examples of pulse-timer relays that should suit your need, to disable the horn-honking after a period of time:

DEI's 528T:
https://www.caraudio-caralarms.com/528T.html

PAC TR-7. I've never used one, but it seems like a really cool part:
https://www.soundoffaudio.com/item.cfm/id/2084

--Also, more than just limiting the duration of the horn..... you also probably want the horn to pulse on and off, rather than a continuous blast......horns are not designed for continuous operation; it might burn out.

For that, I would suggest an electronic (not thermal) turn-signal flasher from an auto parts store. I think they are designed for a positive, high-current trigger......so you may HAVE to get the positive horn wire under the hood, rather than the negative one at the steering column.

--You do know that whatever switch you choose to mount outside the car, would have to be waterproof, right? If you don't object to drilling a hole in your fender, check out the key-operated switches, like the used in the '70s.

--As far as your other switch, and your test light....there is an easier way.

Purchase a 12-volt, flashing LED, and wire it up to receive power & ground whenever your system is "armed."

You can buy such an LED as a fake car alarm from eBay or J.C. Whitney, or just the raw LED from Radio Shack or Parts Express. They are not hard to find.

Not only will you save the trouble of an extra switch, you'll also have an added theft-deterrent when potential thieves see the flashing light.

--Not sure if you know this, but some remote starters have a wire that puts out a ground ALL THE TIME during the time that the doors are locked. This would be to hook up an optional starter kill setup.

Take your circuit tester, and try it out on each of the unused wires from your remote starter, while locking/unlocking the doors with the remote...........if your unit has such an output, it would save you all the trouble of that hidden switch, and be easier to use as well.

--The way you have your relays set up in that diagram looks fine. But just a reminder in case you didn't know it------you mustn't set up a relay that stays on all the time when the car is off; it will eventually drain the battery.

--What are you using to make your wire diagrams? I want to be able to do the same.

thanks for the info. i used Paint to make the diagrams--i've used pspice before but i'm not very good at it. i'm not sure where the remote starter is. It must not be very big because i've taken apart a lot of things on the car and i've never seen it.
There is some sort of battery saver feature as well. When the engine is turned off and the key is taken out, the stereo, windows, etc. will all work for at least 10 minutes, until one of the doors is open. I think it is tripped by the battery getting down to a certain voltage. When i wired in my CB radio, i wired the positive right to the battery and the negative to the frame. The battery saver feature even affects this (CB won't work after battery saver shuts off radio). So i think it might be shutting off the ground path from the battery to the frame.
I think i'm actually going to use an aftermarket horn for this, since i have a few of them laying around. It is the same as one of the factory horns, but the low-tone only.
I think the flashing LED is a good idea. But won't that drain the battery as well?
I think i understand what you are saying about the relays being a current draw...but in the diagram above, they would only draw current when the dome light is on, correct?
I'm not too concerned about somebody actually stealing my car. I live in a pretty low-crime area. Somebody broke in it last week, but I think I left the doors unlocked. Obviously it would be easy to disarm the system if you knew where the switch was (or even that there was a switch). But i'm assuming they won't figure that out until they have already set off the alarm.

Here's something you can try putting together for controlling a relay.
When the switch input goes low, it should turn on the relay for a few seconds, and turn off again even if it stays low.
Relay turns off immediately when the switch input goes high. You can make the duration of the relay on time whatever you want by changing the values of R1 and C1-

Pulse timer