You only need two relays total, not one for each mode for each bulb ... unless these bulbs draw a LOT of current! In that case they would be really inefficient for this use.
Assuming you want to use the OEM switch, high/low beam switch, and wiring, here is what I did on mine and suggest:
Use relays designed for vehicular use. I didn't look up the Radio Shack part numbers, but using ones designed for other purposes may lead to problems in the future (moisture/corrosion). You can get standard automotive plug-in relays nearly everywhere, and they're not terribly expensive. These types are used in many newer vehicle electrical systems and are very common. Usually they are rated at least 30 AMPs.
You can also get sockets for your relays, which makes them easier to change if one of them goes bad. These sockets should be available almost anywhere you can get the relays. Don't forget to use dielectric grease on your connections to keep them from corroding.
Get your high or low beam signal from the wire harness connecting to your headlights. There will be one wire for high, another for low. Connect this to one side of the relay coil (that causes the relay to activate). Connect the other side of the relay coil to ground. On the relays I have, these are labeled 85 & 86.
Then use the Normally Open contacts of the relay to pass power from a FUSED 12 volt source through the relay to the headlight when power is applied. This way, when the relay gets power from the headlight switch and low or high beam switch, it closes it's contacts and passes power through and you get the appropriate beam. If your relay is an SPDT unit BE CAREFUL to put your power in on the side that doesn't connect to the other output when relaxed, otherwise you will have an unused hot lead when the lights are off, which could cause trouble for you. If labeled like mine then you want to connect the 12 V source to pin 87. I'm not sure where these numbers came from, but they are a little funny. The relays I have have a label with the numbers 87 and 87a or something like that for the Normally Open and Normally Closed outputs. You might have to experiment a little to make sure you get connected to the right 87. If the lights are on when the switch is off, you need to change to the other one!
Connect the wire to your headlights to pin 30.
Some of these relays have SPDT contacts, but remember when the relay is in it's relaxed position (not powered), it will still feed power through to the relaxed position contacts, so your lights will be on all the time!!
Size your fuse to whatever current level your headlights draw, then add a little. Since fuses are generally available in 5 amp increments, if your headlights draw 15 amps then you can use a 20 amp fuse. You only need a high enough amperage rating for ONE beam of the headlights, since only one will be on at a time -- unless something is broken. Make sure you size your fuse for the beam that draws the most current. Also keep in mind that generally 12 Gage wire is only good up to 20 AMPs, and 14 Gage is only good for 15 AMPs so make sure your wire is sized correctly. If you are drawing 30A, you should use 10 Gage. Otherwise you will have an unwanted heater under your hood!
I went directly from my fuse to the battery for my 12 V source.
I don't have any fancy drawing software handy right now, so I'll try my best with what I can do here:
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-- High or Low Beam signal (+12 V)- == 85 |
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-------- Ground (-) --------------- == 86 |
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-------- Fused 12V Source -------- == 87 |
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--- To Headlight (NO Connection) -- == 30 |
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----- Unused (NC Connection) -- == |
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Duplicate for each beam, high and low.